Of those 11 channels only 3 are completely separate, in a moment of desperation I would try um all, on the router and the phone is the phone has a channel button, as the phone may be on offset channels.  However I would start by trying 1,6,11, then I would buy a DECT 6.0 phone.  They gave wireless phones their own frequency for a reason, some phones will even have the potential to knock down every router in an apartment building on all channels.  Interference is a major annoyance.