said by DaneJasper:
If any DSLR members might have suggestions for this apartment dweller, who's building wiring is cutting speed by half,
It is not possible to filter in band noise.
You can try the coax with baluns at both ends, not any converters.
Best bet is to put the modem in the closet inside a locked box.
Use the existing pairs for ethernet. Possibly POE to power the modem.