You main router (the one does the DHCP and NAT) should do the work to manage multiple Internet connections.

One Internet connection is your modem. Another will be your phone tethering.

So things to check your Unifi router: 1. Can it manage multiple connections? 2. Can it tether your phone, via USB or wifi?

If it can of course you can keep it. If not you can change it to one GL.iNet router, which can do this.

But if your Unifi router can hand multiple connections, you can just use GL.iNet router to tethering your phone and feed a 2nd Internet to your Unifi.