More explicitly, if you’re wanting to use the gl-inet as just a VPN client for your entire network, take it and plug its wan into your cable/fiber modem, then plug your ASUS wan into the gl-inet LAN. From there you can use 0.0.0.0/0 as your allowed ip range on the client and it will forward all traffic. Your current LAN/wireless setup will stay the same, you can turn off wireless on the gl-inet and I think everything will work like you want it to.

This assumes of course you’re not trying to forward selective devices - you want to forward everything.