I think your rax48 uses different hardware. Open it up and compare with your other device and you will see. Obviously performance might differ.
In your tests you use 20 MHz, which theoretically should have higher range than 160 MHz, but you could try 160 MHz nevertheless as some clients have had some uplink performance issues with 80 MHz channel bandwidths. Maybe 20 MHz channels have also issues.
Another thing to check is if you have set the countrycode. This is very important. Without, performance will be abysmal. You did set it to US. My bad. Should be alright then.