I have an issue with configuring SXT LTE. I have installed it outside and the signal is maximum. Unfortunately I can reach only 17-20 Mbps download while being inside I can easily reach 32-35 Mbps on my iPhone. What am I doing wrong? Mikrotik is with the latest stable firmware.

Any number of things. SXT devices are directional, are you pointing it at a tower? iPhone may be using a better band (such as B28). iPhone also supports CA.

Yes, I am pointing directly at the BTS. Signal is full (-60 to -50 dBm). At this specific BTS there is only LTE 1800 (band 3), 800 (band 20) and UMTS 2100.

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).

I have selected 1800 band as it is the only that works so so. Band 20 is much slower.
This is strange... I have another installation that uses the same LTE modem, which is over 6 km away from the BTS, and it works nicely reaching 25-35 Mbps depending on the time. This SXT is only 300-400 m away from the BTS and 25 is reachable in the night only. Is there any way to firmly switch to the other BTS? Maybe this one is much more occupied with users, as it is in the town center, while the other is on suburbs. However, the town is small anyway, about 23000 inhabitants.

You have answered yourself.

You have answered yourself.

I don't think it's a real reason, but let it be. So how I can connect to the specific BTS? Now it picks up the strongest signal automatically.
There is similar topic described here on forum refering to wAP LTE kit. The last advise is to change MTU from 1500 to 1428. How can that influence on LTE speed?

I think it should be possible to lock LTE modem to some band. But it shouldn't be possible to lock to a particular cell (tower) ... if modem was locked to cell on the same spectrum part (same MNO same band) with worse signal strength (but lower load), it would both receive higher interference from stronger loaded cell and cause higher interference to it as well. The end result would likely be worse than if left unlocked.

Beware that all cells (towers, transmitters) of one MNO operating in same frequency band are using exactly same frequency ... that's different than WiFi where adjacent APs should use different frequencies for best performance.

Cell lock is supported:
https://wiki.mikrotik.com/wiki/Manual:I ... _Cell_lock