I unfortunately can’t really offer much advice here. I configured Wireguard on my phone by essentially copy/pasting the configuration from my laptop and changing the values as necessary like the public key and client IP address. Turned it on, it activated VPN mode in Android and everything started working.
I guess make sure you haven’t mixed up your public/private keys, your server knows about the new device (and is restarted), and your phone is using the right IP address as basic troubleshooting steps.