Have you tried the "Fake It" setting in the WSJT-X setup? It adjusts the radio frequency to allow for a wider passband. An example would be moving the received (and transmit) frequency to allow get around the narrow SSB filter. Opens up using frequencies above 2.8KHz or so.
A good and detailed explanation of the steps to use FT8 (or other) digital modes with the µBITX
