Well obviously it’s not the sub since you’re on your second one with the same issue. Please tell us more about your network is it:

Router modem Combined

Router and Modem separately 

Router and modem with range extenders and/or access points

Mesh network with main router and satellites 

Any variations of the above

It is a router and modem separately. Both brand new just installed the other day.

However

I just fired them up again this morning and the sub worked immediately!?

So this a good news. I will update if the issue happens again. I'm in the middle of a move in and I need my tunes!

If the router/modem was just installed, it was probably duplicate IP addresses that worked themselves out of the system as IP leases expired.  In the future, it is a good thing to do a network refresh after a router change to recycle the IP assignments from the old router.

You might try setting static/reserved IP addresses for all your Sonos using your router’s DHCP settings page.  Power down all Sonos, reboot router and controller then power back up one Sonos at a time, Start with the SLs then the Sub.

If it keeps acting up after that send in a diagnostic and record the ID number. Once it starts working send another diagnostic and contact Sonos support with both numbers. By phone is best but here usually works too.

Well it's been happening again. I'll try to do what you guys have suggested before giving Sonos a call tomorrow. Cheers!