Just checked my advanced DV phone and I can boost the ringtone and also adjust tone - more treble or more bass.
Unfortunately you selected 2 of the less featured DV basic phones instead of 1 advanced phone
Why don't you just connect your existing big button 200 to the phone socket on back of hub and use it as before. If you want phone elsewhere then get dv adapter and position big button where more convenient
Whilst that works if your hub is in a convenient place and the DV adapters are generally fine for us the DECT range was not enough to reach the hub.
If this is the case the only real solution is to change your internal wiring so that it will connect to the hub then all your existing phones carry on working as before.
BT (not open reach) changed our wiring for no charge but this was probably as many years ago the wiring was done incorrectly and so still belonged to BT / openreach. ( The incoming wire went to an extension and then on to the master socket and they own the wiring up to and including the master socket.)