Connecting Netgear RAX80 to another router.

Hi

I live in a rural community with no fibre and poor copper connection to ISP's. Instead we have a provider that uses 4G/5G technology and a small dish on the house to connect to a mast a few miles away which has a fibre connection. This achieves a good internet connection; I get c200Mbps down and 30Mbps up with a ping of c30ms when connected to the router provided by the ISP. 

My problem is this; i used to be able to input the ISP settings into my own router which worked well, they have upgraded the service and now i HAVE to use their router. My RAX80 router has all of my WiFi connected devices set up, so I have connected it to their router via Ethernet and wireless devices work as before. However, it is so much slower. Using the Nighthawk App to test the speed I get 50%-70% of the performance on their cheapo router.

This is not due to loads of connected devices at the point of testing, it is the same with only one device connected.

Is there anything obvious that I should be doing with settings on the RAX80 or another troubleshooting idea I can look at?

Thanks

Jonathan