How are this K+M connected to PC, USB, Bluetooth...? Do you have  the latest Dell Peripheral Manager (8-Nov'22) installed? Run that and see if you can set the mouse wheel side clicks the way you want...
I recently purchased the dell premier multi device wireless keyboard and mouse - KM7321W.   It was working fine for a while, and everything still works fine except for the wheel side clicks.  I updated my windows 10 software and it quit working.  I've googled and tried to death - cannot fix.  Please help.
mouse wheel click issues