Your mini 2 falls into the "nano" category, you're capped at 15 metres (+/- 50 feet).
You've updated to most recent firmware, so check on the safety tab of your main menu: has your maximum altitude been capped? What is your maximum distance set to?
You mention using the RM500 (smart controller). The drone and controller connect and you get telemetry and video feed... Yes?
Are you using the GO4 app controller sub-menu to change the primary remote from the RM500 to the RC-1B?
Does the drone fly? just the drone coupled with the controller? (Ignore the screen component)
The control signal carries the camera video signal within it, so if it flies reliably, there's nothing wrong with the TX/RX RF transmission.
When you...
There have been over 30,000 recorded deaths as a result of commercial and non-commercial air incidents since the first recorded aircraft death in 1903.
Number of minced, dismembered and deceased victims of consumer class drone incidents...