Router for AT&T DSL - Do not believe AT&T when they say a port is open

We have been working with AT&T on a port forwarding problem. They keep telling the customer that the port is open and it does say it is open on the router " PACE PLC 5268AC" but they have no way of telling if it is truly open. They are not able to use tools to test. I would test the port and it was still closed. This went on for a few hours until I got AT&T staff to call the customer and tell them they can not test the port. We are now getting a new router in a few days but they are down until then.

Can we use a third party DSL modems with AT&T?

AT&T’s project lightspeed is how they deliver most DSL/VDSL/Fiber services. Each DSL modem has a set of certs that authenticate against their network. I have read in the past some folks have been able to extract the certs from other modems but the risk is that AT&T would rotate the certs and lose access to the network.

As for port blocking AT&T blocked common inbound ports here 80, 443, SMTP and few other exploited ports by bots and other machine infections. Not only were the ports blocked on the VDSL gateway but also they had to change my account to a different profile since their head end router was also blocking the ports to this client.