Welcome to AAC!
If the Tx and Rx are in fixed positions you could use directional antennas.
Increasing the Tx strength (if legal/possible) would increase range.

Not to rain on anyone’s parade, but referring to Section 15 of the FCC’s rules, you must use the antenna that was used to demonstrate compliance with the FCC’s rules. In other words the antenna that was installed on the unit.

I hope this gives a little insight.
KD - N5KS

 

RE:""What can I do to boost the range""
1) apply the better modulation algorithm adapting the power and duplicating the "telegram" until it had been declared "read". There are two of such high distance protocols, one is more energy savvy at 2400 MHz namely Nordic NRF024L and another is more energy demanding but three-fold far-beating 868 MHz Lora(TM). Distances are roughly 1 km and 3 km.
2) apply the directional antenna like Yagi-Uda. If not helpful, apply the more vibrators on Yagi-Uda. Adjusted VNA tested dimensions may look here at my springtime posts this year.
3) apply the antenna with more squeezed horizontal lobe structure (not illuminating the Cosmos) like J-pole.
4) apply the output power booster cascade
5) apply the good quality surplus LNA cascade in input
6) apply the max permitted Vcc

 

I think it's been always like that.