Hello, this is Martín, Tech Support Engineer for Teltonika Networks.
The main difference between a Teltonika RUTX12 and a RUTX14 is in the modems: the RUTX12 has two CAT6 modems, while the RUTX14 has one CAT12 modem.
This allows the RUTX12 to have two 300Mbps LTE links, totaling a maximum troughput of 600Mbps. This speed can be reached when coupled with features such as Load Balancing, but each single modem can reach a maximum speed of 300Mbps.
In contrast, the RUTX14 has one CAT12 modem, which has a maximum speed of 600Mbps without the need of Load Balancing.
Also, if you need to perform a deeper comparison between devices, the Teltonika Networks Product Page has a comparison feature.
I have also attached a link for the comparison between the RUTX12 and the RUTX14.
I hope this answers your question, if not, I remain attentive to any further comments.