New phone wont stay connected to wifi

Optimize the range of the mobile network, see this Apple article:

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

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

Read again -> Related Article -> If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network

Check your router settings -> Recommended settings for Wi-Fi routers and access points

Recommended settings for Wi-Fi routers and access points - Apple Support

If they are On, turn Off Hotspot & VPN.