How do you connect 56k fax modem with DSL?
The short answer is that you don't. Sure, you can plug a fax
modem into a phone line that supports DSL, but it is just using the
dial-up capabilities of the line and not the DSL capabilities. A
regular modem like that functions like a telephone and transmits in
the audible frequency range and not the higher DSL frequencies.
With DSL, you'd use a DSL modem (with or without an included
router), not a dial-up modem.