Why is the system *still* not finding speakers and droping out??

Did you unplug/power down all your Sonos before rebooting the router, then wait two minutes after rebooting it before powering back on your Sonos?

Networks are also affected by outside sources. Have you read the wifi interference FAQ?

Aside from reinstalling everything, which rarely helps most issues, have you done anything else to improve stability?

How are your Sonos connected?

I have had the same issue that you are describing. Even though I add my speakers to my system by the next day the Sonos app would say there were bo speakers and that I would have to update the wireless network which may or may not actually bring the speakers back online. 

I came across a vague Sonos community post where someone said that assigned their speakers fixed IP address in their router settings. Fur security reasons the default setting is DHCP which randomizes which Ip address if assigned a device on your network. Now that all my speakers has static or fixed IP address in my router software settings my Speakers ALWAYS show up.

I used Chat-Gpt to walk me through the changes on my router. Be advised that there could be network security issues if you do this. 

The only network security issues you could see is if you disabled the normal firewall that protects internal devices from access by outside systems. Reserved addresses or not that is a bad thing to do, but reservations make it no worse. Most folks only have about 250 possible addresses, how many fractions of a second does it take an attacker to check them all.

IP addresses are often retained, both by cached address information and on newer hardware an attempt to always assign a device the same address, without requiring a reservation. Seems the latter option doesn't always work to cure Sonos related issues and a reservation is required.

Rather than trusting very unreliable AI tools, just use your router's manual or the internal help pages and you won't be led astray by poorly trained AI.

Allocated ip addresses is also suggested by a Sonos rep in this recent thread:
 

hi ​@botany , I am using various SONOS systems and have not observed that reserved IPs for the SONOS devices made the connections more stable - I do have in each system at least one device wired which I believe helps - most likely you have some interference from other networks and/or bluetooth devices - I would check your router for TX and RCV network errors - besides the network channels, there might be other settings to be tweaked to improve your WiFi performance  

Reserving addresses is a solution to a specific problem  that only impacts what appears to be a small number of Sonos systems. If you aren't having the loss of function on reboot or power-cycle problem then it doesn't apply to you.

Wiring a Sonos device creates a Sonosnet (close to wifi) and all compatible Sonos will use it instead of your wifi, maybe not a good thing as Sonosnet may not preform as well as newer wifi systems. There are good support articles to help decide which option is best but Sonos recommends wifi as the best option.