Update: Well I tried turning off all breakers in the trailer, I also
tried reversing the plug. No help.
I asked one of the MRI physicists if anything might intefere with the
DSL in the 1400 KHz range. He said no.
I still had an old DSL modem that was previously thought to be
defective by the contractor. I plugged it in and it worked. Now
everything is fine.
The only explanation is that these 2 modems are made by different
manufacturers and there are probably different tolerance levels
between the 2 involving interference.
Thanks for everyones help.