I'm not sure that I understand what you mean by "RJ45 cabling". In my nearly 30 years in this business, I've never heard of that.
If you hear faint dial tone at the network interface, I would tend to think that you have not actually disconnected your inside wiring while testing. If your inside wiring truly was disconnected during testing, then the fault lies with at&t.
Yes, you can run wiring up to the attic and tie back into the existing wiring. Of course, there is a right and wrong way to splice telephone wiring, so using wire nuts or other electrical splicing devices are not the way to go.
Lastly, make sure that you have DSL filters for all other phones/devices on the line. The only device that should be connected directly to the line is the DSL modem itself.
Hope that this points you in the right direction. Welcome to the forum!
