56 Kbpsis not a connection speed that a modem can operate at. Modem connection speeds show one of two values - the actual connection speed, or the speed setting of the comm port.
Now 56Kbps modems cannot operate as fast as 56Kbps (they are limited by line characteristics, and the absolute maximumis around 52-53Kbps - typical would be 45-50 Kbps - sometimes slower in poor situations). Thus your old modem must have been showing the comm port speed (normally would be 115Kbps or 56Kbps)
A V92 modem will operate at V90 if there is not a V92 connection available - so there's nothing to add to alter it.
The problem is, has your line always been like this, but you couldn't see it, or is the modem really slower - it's not possible to tell now.
The best check of whether it's your system and/or house is to go to another location with your system, where you know a faster speed is normally obtained, and test it there.