At this specific BTS there is only LTE 1800 (band 3), 800 (band 20) and UMTS 2100.
Check which is used LTE band while your data rate is slow. B20 cells have generally lower capacity but (due to better propagation of lower frequencies) higher signal level, so devices might select this band. As @RogerWilco wrote, modern LTE devices support CA (Carrier Aggregation combining data rates from different frequency bands), but devices from current MT LTE roster don't.
Forcing RBSXTR to use B3 may give you better performance or not (depends on B3 cell signal level and cell load).