Setup home vpn server

Hi everyone, I just bought a GL-MT300N-V2. I need to turn on my home PC remotely.
I have a main modem and the GL-MT300N-V2 connected to the modem via ethernet. If I connect to the GL-MT300N-V2 wifi it works.
I have tried to configure WireGuard Server on the GL-MT300N and export the configuration to my smartphone but it doesn’t work.
I have a public ip. The ip is not static but I can use dyndns.
I set up port forwarding on my main modem (from main modem to GL-MT300N-V2)

My smartphone is mobile data only.
Not working.
(tried same setup for openvpn server, same result)
Am i missing something important? Thanks for the help!

I’m confused about your configuration. You refer to the Mango, a “main modem”, a “main router”, and a home PC. You have set up port forwarding on the main router, but you don’t say from what to what, and you have the Mango plugged into the modem, not the router.

Start with a configuration without the main router. The Mango WAN port is connected to the modem LAN port; hopefully when it turns on and you connect to the Mango wirelessly, the Mango WAN port pulls your public IP (that is, the modem is in bridge mode). Then connect the rest of your devices wirelessly to the Mango or to the Mango LAN port. Now you should be able to set up an OpenVPN or Wireguard server on the Mango, without port forwarding, and access the Mango’s LAN, including your PC.

Now introduce the main router in place of the Mango. Plug the Mango into the main router. If you do the port forwarding on the main router, you should be able to make your OpenVPN or Wireguard connection. But now you see you have two LANs: one that is run by the main router for devices plugged into it, and one run by the Mango for devices plugged into it. If your PC is still connected to the Mango, you should be able to reach it, but not devices plugged into the main router, without more work. If you don’t have other devices, there is little point to the main router.