mandrake The IP address needs to be identified on both devices. If your laptop is Linux, go to terminal/CLI and enter:
It will return an IP address. Focus on the first 3 numbers, the fourth will be the device.
On your mobile, there are two methods. Either, go to:
Settings, About, IP Address, or
Settings, Internet, Wifi (you need to connect), scroll down to IP Address.
Check these IP addresses without VPN switched on, see what the numbers are, and then check with VPN switched on - either through your Proton VPN account on your router or phone, and then with VPN switched off. You mention split tunnelling, which means two different IP addresses. Hopefully one of these will be the same three numbers as your laptop.
Do I assume you are using Linux (per KDE Connect) and not Windows or Mac?