There should be no problems behind a Salt/Sunrise modem. I operate the device behind a Swisscom modem and it works perfectly. You will need to check with your future provider whether you can operate it directly via the SFP module. I don't know the details at Salt and Sunrise, but they can connect it at Swisscom. The SFP module must be registered with the supplier beforehand. But as mentioned, it also works behind a modem.