When logging into the web interface on the phone under
Network > Advanced
There should be a SNMP option
However this is not present on these phones running the 3CX firmware
How can we enable SNMP?
I believe that SNMP was removed by Yealink a few years from all firmware but i am not certain if this affects all phones. I would recommend asking this question in the Yealink forums as you might get better answers there.