I am on area, where my mobile operator (Plus in Poland) provides UMTS2100 and UMTS900 signal, but UMTS2100 is working only as HSDPA 3,6 Mb/s, however UMTS900 is working as HSPA+ MIMO 28,8 Mb/s (category 16). I want to use naturally faster UMTS900, but network configuration is such that the priority is to UMTS2100. So is there any way to force UMTS900 or UMTS carrier 2963 on N900? Actually the only way to use UMTS900 on area, where I am, is significant weakening of the signal, by strong antenna cover, because when UMTS2100 is below -105 dBm, occurs handover to stronger UMTS900, but still after a few minutes phone returns to UMTS2100. Difference in transmission speed between UMTS2100 and UMTS900:
UMTS2100:

UMTS900:

Tests done on N900 connected to computer via USB cable.
@edit UMTS900 upload speed on speedtest is not real, because I use on computer cFosSpeed, that didn't calibrate to HSUPA speed. Real upload is about 1,5 Mb/s.

go into settings - phone - select 3g - save or download 2g/3g swicher and in the drop down status menu you can change from 2g 3g

It forces only UMTS, but not UMTS band or carrier.

Sorry for my bad english...

Forcing the 900MHz band is not possible and would be not convenient at this time.
It is not possible because they are already defined as UMTS channels, so as well can't force the DCS band in GSM phones (unless in some specifically modified).
Forcing only UMTS900 band would still not convenient because the channels are few, since they are still used by GSM and then you might not have sufficient radio coverage.

If so, is possible to force UMTS carrier on N900?

No, not until now. The phone module is locked.

I accessed phone modem by 'sudo pnatd' command, but known for me AT commands for forcing UMTS900 or UMTS carrier is not working.