Phone your company that provided the modem, explain your problem and ask/demand a modem that complies with the code, that is how it works here, on my side of the pond all equipment must comply with the minimum CE code, and withstand 20 volt per meter H.F.
Some modems here from cable or ADSL can give problems, but cable and ADSL providers will swap them for better ones if costumers complain.
I caan make 1000 watts in any antenna, with the modem and router 2 feet away from my gear, it's behind the cabinet my hamgear and amplifier is situated in.

No problems too with neighbous running that power too.
If you used an dummy load your modem is your problem, so contact the firm that provided it.