How are the receiver and antenna wire mounted? That can make a difference. Get the antenna as vertical and high as possible. Preferably away from other wiring and electrical stuff. Make sure there is no damage to the antenna wire. Don’t crimp/bend the end of the wire over with a cap, use a longer antenna tube.

Is the range inadequate in all driving conditions, or perhaps just when punching the throttle? If just when punched, could be the voltage to the receiver is sagging too low momentarily. A cap on the receiver (aka stutter stopper/glitch buster) can help, or an external BEC setup, last resort a receiver battery.

Suspect there is a solution. I don’t do any of the above stuff, yet I’ve given up on doing a range check with any of our Spektrums, I can’t see far enough. But I am rather old and creaky.