Dim Pro uses Elements. There are 4 Elements available with each instance and each Element can have it's own patch and midi channel. The problem with using it with Biab is Biab only has one midi port so you can only use one instance of Dim Pro. That means a maximum of 4 instruments. That could be enough for a lot of styles though and is plenty if you're using some Real Tracks or the Real Drums. I think your problem is you're not assigning the correct midi channel to each Element. As was pointed out above, it's gets tedious to have to go into Dim Pro and load a different patch every time you want to change an instrument in Biab. That's why most of us use use a dedicated GM synth because all the proper instruments are already loaded and ready for Biab to call them up.
Real Band is a different story. It has 16 midi ports so you could load 4 instances of Dim Pro and have 16 instruments available on 16 different midi channels.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.