I’m running Ubuntu 20.04.2 as my Wireguard server with several GL iNet clients that are running a mixture of 3.10x and 3.201 firmware that are working along with Windows and Android Wireguard clients, so it should be able to work for you.
Without seeing the Wireguard config files from both your Ubuntu Wireguard server and your GL iNet client, it is going to be really hard to tell you what is wrong (make sure to remove any private info, such as keys, before posting to this forum). Also, is your Ubuntu server a physical server or a cloud server. Is your server on a local address or an external IP address? Lastly, can you ssh into your GL-MT300N-V2 and then from the GL-MT300N-V2 ssh into your Ubuntu server, just to make sure the GL-MT300N-V2 and your Ubuntu server can communicate?
So there are no peer entries in your /etc/wireguard/wg0.conf file?
With what you have posted, nothing is jumping out as an issue. There are several post posts of issues with wireguard not working after the 3.201 upgrade if you kept your config. I would clear the config on the router, reconfigure it, and try it again.