Yes, it's possible and lot of vendors have already done it that way. Your select service/pay bill page can take the credentials and either it can post directly from client to AP or it can post to your external webserver and then your external webserver does a post to AP. If you notice the query strings has a ga_srvr field which has the AP IP address and your external webserver can use this IP to frame a POST request directly to AP.
There is a latest document available on cambium support website on how to integrate external webportals with cambium AP. The below link can take you to the specific page where you will find the document on "Guest Access Portal Integration"
https://support.cambiumnetworks.com/files/e400/
I have a prototype webserver written in nodejs which you can try to play around to get to know the whole flow with external webserver. Please let me know if you would like to have that nodejs webserver.
Best Regards,
Kunal