Hi, your attenuation and noise margins are just fine for 2.5km of 0.4mm cable, your sync is probably abit under the mark, two things could be the cause of this, firstly your cr@p old modem, these Dlinks have long since gone the way of the dinasor and house wiring, if you have any splits or taps int he cable to more than one socket then to get optimal results you will need a master filter to stop echos from those stub tails but as you have already installed a master filter that can be discounted, so try another modem, TP-Link is good.

Cyril