Cable Modem to DSL. Not quite sure what you are trying to do there since DSL assumes you have a separate modem
Generally a reset to factory will sort most things. Should be a little button on the back to press in with something like a pencil for a few seconds with the power on.
Whatever modem you are connecting to, make sure the power is off on both modem and router, when you swap the connections over.
A "B" speed wireless router is quite old these days. I'd be tempted to be putting it in the bin since most devices that can connect wireless will these days be at least "G" speed.