Okay, so it does seem this device doesn't implement Mask Write Registers:
2024-02-28 15:45:18:193 drivers-modbusdriver2-BHD-1 Received response with ExceptionCode: 0x01 (IllegalFunction). unitId=1, referenceAddress=1499
2024-02-28 15:51:21:055 drivers-modbusdriver2-BHD-2 Received response with ExceptionCode: 0x01 (IllegalFunction). unitId=1, referenceAddress=1500
Unfortunately this means a bit of work to get around it... you have to assemble any bits you want to write into a whole word and then write that word to the register. You can create a Derived Tag for each bit to make things a little easier.
This approach is described here: Troubles making a Derived Boolean Tag work as a mask for a Short Tag - #2 by Kevin.Herron