As @admon points out, they can block it if the network is using network attached packet analyzers. Those devices can distinguish that the encrypted traffic is not using HTTPS (SSL) encryption so they could block it if it isn’t.
Anyway, I was able to do it on two gl.inet devices running version 4.1 firmware before upgrading one of the devices to gl.inet firmware version 4.5 which now has the settings available in the GUI to change the Web GUI TCP/443 port so that TCP/443 could be used on the OpenVPN server instead. Check out the below thread for setting the OpenVPN TCP port manually on gl.inet firmware versions before version 4.5 to see if it’ll work on your router.