Vivo 5G :;Has Anyone Found a Fix?

Vivo phone showing network errors across apps, plus Vivo account verification problems. Has anyone fixed this?

I'm trying to help my dad with his Vivo 5G phone running Funtouch OS 15 on Android 15.

Google Play Store, Vivo Store, YouTube and system updates show network errors, even though WhatsApp receives messages and Chrome can search Google. The phone also showed "ERR_CERT_AUTHORITY_INVALID" and a security certificate warning on the Vivo password recovery page.

We've tried network resets, changing Private DNS to Automatic, restarting the phone and clearing app caches. The issue persists on WiFi and a mobile hotspot.

We reset the Vivo account password, but login sometimes says “Network unavailable” and sometimes “Incorrect password.” We managed to log in from a laptop, but the verification code isn't arriving, and the registered email is partially hidden, so we don't know which address it is.

Has anyone experienced these problems on a Vivo phone? Did an official software reinstall fix it, or was there another solution? I'd appreciate any advice.