Have you tried switch off WiFi and mobile data on phone Aset up a
hotspot on phone AUse another phone B running Google home and connected
via WiFi to phone AReset your speakers and connect them in the home app
on phone BIf they work it's your WiFi
I had exactly this problem and it was down to poor WiFi signal in some
areas. I bought a couple of Xiaomi ax1800 routers, set them up as a
meshed network and since then it's a worked fine.