From your British mini 2SE? The OP is from India. Different regulatory ball game... that's why Drone-Hacks and other de-restrictors include a workaround altitude hack specifically for mini drones used in India.
And don't forget that about 3...
Not an issue? Even though you're saying that it's "...getting stuck at 20-25 metres and it's freaking me out..."?
The fact that it's started happening recently and after a firmware update might suggest that certain custom settings and parameters...
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...