Try rebooting your router and phone. You can also try deleting and re-installing the Sonos app.

Seems odd, I’m using 15.4.1 on both an iPad and an iPhone, and both run the Sonos app without issue, including finding all of my speakers.

I’d certainly second the “reboot” thing….and perhaps share additional information about which version of the Sonos app you’re attempting to run. As well as how your Sonos speakers are connected to your system, via either wifi, or SonosNet. Wouldn’t hurt to know the make and model of the router in use, either, as well as whether it’s a mesh network setup, or a standard network (single point). 

In fact, I’d be tempted to suggest unplugging all the Sonos devices from power before rebooting the router, just as an extra precaution, to cause a full reload of the software on each device, as well as a refresh of the DHCP table. 

All working okay here too on iOS 15.4.1 both for iPad and iPhone XR S2 Sonos controller App .

I had same problem.  Updating Sonos S1 app from version 11.3 to 11.4 fixed it for me.

S2 app, iPhone 15.4.1

I installed Google Nest Wi-Fi mesh router and now nothing works. I’ve tried rebooting every hardware, factory reset, delete app, create new system, etc… my iPhone is unable to connect to network “SONOS” when it tries to pair and times out. Very frustrating, wasted an entire Saturday. All my other home devices, smart gadgets etc work fine on new network.

The key part of your post is ‘mesh’. Sonos requires both the controller and the speakers to be on the same subnet to be able to talk to each other. Wire one of your Sonos devices to your base of the Google mesh system, wait a couple of minutes, then open your controller. 

Also make sure the Sonos app still has “local network” permissions, which is sometimes lost after an upgrade.