On some of my Ubuntu based VPN servers I use the SoftEther VPN package which provides OpenVPN along with multiple other VPN protocols. I have had problems with some versions of GL iNet router firmware not being able to connect to the SoftEther OpenVPN servers, where Android, Windows 10, Macos, and Ubuntu OpenVPN clients work just fine with the SoftEther OpenVPN server.

I could never figure it out, so I also installed the native OpenVPN package on my Ubuntu servers using the angristan script on a different port, and it works fine with the GL iNet OpenVPN clients, and all other OpenVPN clients. I cannot tell you what it is, but I have definitely seen something similar to what you are experiencing, where the GL iNet OpenVPN client fails, but all other OpenVPN clients work.