Re: dsPIC33EP512MU810 uart pins
(I didn't even look into the datasheet, so no references...)
If it's got that many UARTs, some if not all have their I/O pins assigned via the PPS configuration registers. So look into the chapters U(S)ART and PPS.
PEBKAC / EBKAC / POBCAK / PICNIC (eventually see en.wikipedia.org)