You need to select that coil in both the remote AND the headphones.
First of all, make sure you entered the correct serial number and did not mix up a letter with a number,, etc..
Then ensure the that coil is being seen by the remote by selecting that coil from the coil menu (Presumably coil 2) and ensuring the remote's speaker is on (the easiest way to do this is cycle power on the remote, then select yes when the "Use Speaker" prompt pops up, the co and select Coil 2). Then test the coil with a test target waived over the coil.
If it is entered as Coil 2 in the remote then go to the headphones and navigate the settings menu until you come to coil and select 2 (or whatever slot you put the x35 coil in on the remote). Let it connect and then see if you can hear it.
Whenever you swap coils in the future, you need to make sure the new coil selected in the remote AND headphones.
HTH