There is another more complicated guide for PureVPN:

I think the .ovpn files to use are for Linux OVPN Version 2.0:

https://s3-us-west-1.amazonaws.com/heartbleed/windows/New+OVPN+Files.zip

I used PureVPN successfully for about 10 years, but switched to NordVPN about 2 years ago. NordVPN OpenVPN works well on both PC and GL.iNet.

If you are looking for a new VPN provider, then you may consider a Wireguard service such as Mullvad, Azirevpn, VPN.ac, etc. because the throughput will be much higher on your GL-XE300. Although NordVPN has Wireguard, it only works on their apps and they do not provide configs for routers.