This happened to me also. On FPBX 13 after upgrading from 12. Enabled WebRTC in user manager and then enabled DTLS settings in extension. This allowed webRTC phone to work well, but broke the hard yealink endpoint that shared the extension being used in the UCP and webRTC. Had to delete the extension in FPBX, and re-create it, and then the yealink extension worked again. The error I was getting before doing this even after disabling DTLS on this extension in FPBS was:
chan_sip.c: Failed to receive SDP offer/answer with required SRTP crypto attributes for audio
No changing of settings for the extension in FPBX gui fixed this behavior and no changes on the endpoint mattered either including a reset to factory defaults. I am using the FreePBX distribution and commercial EPM module also.