Your issue description doesn't provide me enough information to make a product suggestion, but here are few points you should consider.
You are using a 2500 foot 25 pair cable, but you didn't provide gauge/cable type? However you also state 8 other T1 circuits are working except for one with a radio relay. Does this circuit TOK to the extended demark/handoff with a T1 test set? If so, perhaps the issue is with the Radio relay equipment i.e. some sort of option or perhaps there is a delay issue.
Another point to consider with T1 service on regular Telco 25 pair cable pair separation between the XT and XR pairs is required i.e. you shouldn't use consecutive pairs for XT and XR for a circuit.
If this reply doesn't help, please contact the Adtran Product Support at 888-423-8726 and open a ticket.
If the NI is a ADTRAN HDSLx Remote unit the customer handoff is a DS1 signal. If the 2500 foot 25 pair cable doesn't allow the circuit to work your only option is to try a DS1 repeater between the NI and the customer premise. ADTRAN doesn't make conventional DS1 239, T200, or T400 repeaters.
Your only other option would be to try building another ADTRAN HDSL2 circuit with a locally powered T200 NCTE Mount equipped with T200 H2TU-C P/N 1223006Lx connected to a standard H2TU-R P/N 1223026Lx in a T200 NCTE Mount. Essentially this is a back to back HDSLx circuits which might have too much delay for the Radio Relay equipment, but maybe worth a shot.