not the 1st time they’ve lost their way

Most likely your Bell router is holding on to some duplicate IP assignments, which would cause this same issue in either wireless or wired systems. 
 

Try powering down your Sonos devices, and while they’re off, reboot the router. Give the router a couple of minutes to finish loading before powering back on your Sonos. That should fix the issue, at least temporarily.
 

Once you’ve done that, look through the router’s instructions on how to set up reserved IP addresses for all your Sonos devices, which will work to keep this issue from occurring again. 

Maybe, but I’m on my 4th internet provider in 3 years ( Xplornet satellite, Rogers LTE, Starlink, Bell Fibe) and this is the first time I’ve had a problem when my router reboots.

Sonos isn’t recovering gracefully when Bell Gigahub has a power outage. It requires a power off at a minimum and a complete factory reset on bad days.

Way more work than I signed up for and more than the non-techies in our house will ever figure out when I’m gone.

This suggests that there is a router issue, rather than a SONOS issue. Reserve addresses as Airgetlam suggests. An amazing number of difficult to describe issues with “perfect” networks disappear after this is done.

I suggest that you refrain from Factory Reset without further consult. The reset destroys diagnostic data that may have been useful to discover the root problem. As you have discovered, Factory Reset does not usually resolve fundamental issues. 

I had to be dragged (kicking and screaming) into the reserve IP addresses camp.

I tried many other things and spent hours testing and digging in router logs. Got nowhere.

Took 10 minutes to set the reserved IPs, rebooted all and the power/update/reboot issue was gone.

Ditto. (Actually not kicking and screaming: I was looking for help with my troublesome system; this was suggested.)

For the time it takes to do, surely it’s worth a try, @Tim8aBug?

Ok, I tried the kicking and screaming, it didn’t work for me either. 

I’ve reserved the IP addy’s and hopefully the two Moves will recover after the next router outage/reset. If I remember, I’ll post the results when it happens again.

Thanks to everyone for the tip!

As  fate would have it I had reason to disconnect the Bell Giga Hub router this morning. When I powered it up again and loaded the Sonos app it wouldn't connect to the speakers but it did  instruct me to power them off and on after which they all worked, so I consider the static IP suggestion a win. 
 

As a side note, turning a Sonos beam off and on is an inconvenience when it's plugged in behind a TV cabinet. I found this to be one of the best uses of a smart plug for those of you in a similar situation. 
 

Thanks again. 

Are your SONOS speakers wired to the network or are they wireless? If they are wired is there a network switch between the router and speakers? If they are wired and you have a switch, you may be able to work around whatever mess the router is throwing at the system by momentarily powering down the switch or disconnecting the player feeds. If this works it will save you the trouble of physically visiting each player. There may be another dimension:

How long after the router shutdown and restore did you attempt to use the SONOS system? I can imagine a clumsy router startup that could leave the SONOS system starved for an IP address. Similar to instructing the kids to go to a prearranged spot and wait for adult supervision if there is a general emergency, the SONOS system may run off to a safe network space. In the SONOS case, approximately every half hour the units will check to see if the emergency has passed and they will rejoin the network if they can find it. This can result in a frustrated user because SONOS might recover from middle of the night router update mayhem and the user will never notice, however, a daytime update followed by an immediate attempt to use the SONOS system might run into trouble. If your SONOS system is wired directly to the router, try inserting a network switch between the router and SONOS system. Even though this may not be necessary from a connection count standpoint, the switch will remain powered and this will fool the SONOS units into thinking that network service has simply slowed and there is no need to run off to the emergency safe space and wait for an adult to stop by and reset everything.

@buzz they are all wireless, 2 Moves and 1 beam. I attempt to use the speakers a few minutes after the router reboot when other devices are successfully working e.g. Amazon echo’s. I don’t mind powering them off and on as long as that works. My issue was that it didn’t work several times and the only way to get the app to even see the speakers again was to perform a factory reset. That problem appears to be resolved with the static IP's. I’m satisfied with the ways they’re working now. 

I’m updating this because someone might find it useful.
 

When I tried to connect to my Sonos Move’s from my iPad today, S2 couldn’t find them and wanted me to go through the “Let’s fix it” process which always leads me through a lengthy reboot of the router and the Moves. Instead of doing that I went to my iPhone and lo and behold I was able to control the Moves. At that point I went to the Bell GigaHub to check the Sonos connections and I found the iPhone and Moves were connected to the 5GHz band while the iPad was on 2.4GHz. The Beam was also on 2.4GHz. 

So I turned off the 5GHz band on the GigaHub and everything switched to the 2.4GHz band and now I can control the Moves from both devices. 

In the past I’d created two separate SSID’s on the GigaHub and connected everything to the 2.4GHz band but the GigaHub likes to update itself, unannounced, and it always reverts to a single SSID for both bands. Sonos S2 doesn’t seem to like that and stops working if the speakers are using a different band.
Hopefully the 5GHz band will stay off when the GigaHub updates itself, although I highly doubt it. At least I now know what causes the problem and how to work around it.