Moving RG with longer cable, but no connection

Hello! I have an interesting situation and hope the gurus here have an answer. I am a U-Verse customer, voice and Internet only. Modem is a 3801HGV. When it was installed, AT&T placed the RG in the room where the phone lines attached to my house. New wire was run from the terminal box to the pole, and a Cat-5 line from the terminal box into my room. The problem: the RG’s wireless is too weak to completely cover my house. I’ve tried using signal repeaters, but I would rather have a single wireless. I want to move the RG to be more central. I bought a 25’ Amphenol Cat-5 with RJ14 ends. I tested the cable with a 2-line phone and the pairs have continuity. But… when I use this cable with my RG, the RG cannot establish connection. The broadband light flashes green, then red, then the unit restarts. So to test more: I have two 10’ 2-pair cables I’ve used with the RG before. Each cable works fine between the wall jack and the RG. But if I put them together (using a cat-5 coupler) the modem cannot establish a connection. Same symptoms as the 25’ cable. When U-Verse was first installed, the tech told me I was on the edge at 2900 feet or so. I only ordered Internet and voice, and during the installation he strongly recommended I do not attempt to use the TV product. I normally do not have any problems with my connection, 25 down, 2 up. So my question: where can I go from here? I’m guessing the extra cable length is just “too far” but adding 10~15 feet seems to be cutting things too close. What tests can I run to narrow down the issue? Thanks!