2100 MHz 3G/4G are HSDPA/LTE so your phone needs to support HSDPA/LTE as well as being GSM... and then you also get 2100 MHz LTE on CDMA...
On 2G, 2100MHz was only on CDMA, which most of the world (still) doesn't support... so if you've got a CDMA phone and are travelling anywhere outside of a very limited list of some 45 countries (North/Central/South America, Caribbean, Far East), you're SOL.
So like I said UMTS/HSDPA/HSUPA gets more complicated, because then you're not just talking GSM or CDMA... your phone needs to talk UMTS/HSDPA/HSUPA as well as having the correct frequencies and GSM or CDMA.
On 2G, 2100MHz was only on CDMA, which most of the world (still) doesn't support... so if you've got a CDMA phone and are travelling anywhere outside of a very limited list of some 45 countries (North/Central/South America, Caribbean, Far East), you're SOL.
So like I said UMTS/HSDPA/HSUPA gets more complicated, because then you're not just talking GSM or CDMA... your phone needs to talk UMTS/HSDPA/HSUPA as well as having the correct frequencies and GSM or CDMA.