Just set up the ELRS receiver as per the documentation and in Messages you will see something regarding the RC input instead of No RC
And the radio calibration screen in MissionPlanner will produce results.
https://ardupilot.org/copter/docs/common-tbs-rc.html#crossfire-and-elrs-rc-systems
and
https://ardupilot.org/copter/docs/common-crsf-telemetry.html#tbs-crossfire-telemetry