That would have been like 7 years ago so not sure of the original sources, but for example this mentions it:
The P1,S1 & S2 Chipsets are custom Asics used by DJI in their Ocusync based drones that are the basis for their Ocusync RF Wireless Link. These chipsets are similar to early Leadcore SOC’s and are believed to be direct “descendants”. The P1,S1 and S2 chipsets have only ever been found in DJI...
fpvwiki.co.uk
Basically they used phone SoCs so they did have LTE hardware at hand, AFAIK now they replicate it using SDRs.