I have an Infinity looper, if you’re still having issues (after setting the ModFactor to XMT).  The Infinity should sync automatically when it receives MIDI Clock.  It’ll start sync’ing at the next full measure [default: 4/4].

If you adjust the tempo BPM wildly on the MF, the Infinity will gradually adjust to the new tempo setting (by design).  If you lose MIDI Clock to it – for any reason – the Infinity will try to hold that last sensed tempo internally.  Eventually, it will begin to drift.