I use the same straight-thru rj11 faceplates from the A&A shop.
I had a very bad performance experience with some nasty ebay cables made out of STP CATx with RJ11 plugs put on by hand v badly, the twist wasn't maintained right up to the ends. Changing to the kenable ebay ones (I think), 0.5 m, made a very important improvement in DS sync rate, >20%.
But we digress, as this is would be the same for both modems, so while these are nice tips, I can't see how this addresses the issue.
Unless...
Mains cables touching / near to one of the rj11-rj11 cables, or the modem-to-router ethernet cable?