IOS update

Restart the phone.

 Reset the router to factory settings (menu or paper clip)

On iphone, Settings, General, Transfer iphone or reset, reset, reset network settings

Uf you have a VPN you are strongly recommended to uninstall it and its profile in Settings, General, VPN and device management 

See these Apple articles below:

If you see No Service, Searching, or SOS on your iPhone or iPad

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

Update your network provider settings manually on your iPhone or iPad

Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support