Hi ... I too have had issues with the Airport Extreme and the Xplornet 4G fixed wireless network (Eastern Ontario). After several attempts to have a sensible conversation with Xplornet, I finally found an Apple support tech who has a solution to Xplornet's problem. Here is what the Apple guy had me do:
In airport utility, double click on the Extreme icon, click Edit, select the network tab (router Mode should be DHCP and NAT), select the Network tab (Network should be Create a wireless network), select the Internet tab >>> here's the magic >>> set DNS Servers to 220.127.116.11 and the next unlabeled field to 18.104.22.168
These DNS servers are built by Google and are free to use. The Apple guy was quite enthusiastic about them!
He could not speculate as to the nature of the actual problem, but this solution works. I now have all 10 of my devices connected to the internet with IP address of 10.0.1.x, as they should be. It would seem that Xplornet has an issue with their 4G implementation, but refuse to even investigate. Thanks APPLE!!