Posting this info hoping it will help someone. It would have saved me at least a week of searches, emails and phone calls.
We upsized the alloys on our XC60 MK2 2018 from the stock 18 inch to 20 inch and with the wheels and tyres we were also mis-sold a set of TPMS in-wheel sensors.
Pretty happy with the sale until I started looking into how to sync up the new TPMS sensors with the car. The car kept kicking off thinking the tyre pressure was low. No one had a clue how it's done including the seller and a Volvo dealer I contacted.
It turns out the newer Volvos use the ABS sensors to monitor the wheel's diameter. They do not use the old sensor in each wheel system. There's nothing to sync or relearn in the old way of doing TPMS monitoring. Just inflate the new tyres to the right pressure, turn on the ignition while in park (as if you're almost ready to drive away) swipe right on the main Sensus screen and go to the Car Status icon. Go to TPMS - Calibrate. The dots indicating each sensor need to be green for Calibrate not to be greyed out. My car took a while and it finished the calibration process on route to work.
Exact same process if you rotate your wheels for even wear.

 

best way to tell which system you have is to a) look at the valve stems - rubber is ABS sensor based, metal have the in wheel sensor or b) check what year your car is - Volvo went from in wheel sensors to ABS based in 2015.

Thanks for share on the calibrate process. To note, one drawback I've found on the ABS based TPMS is driving in deep snow can set off a false positive. Had that happen a few times on the wife's Highlander.