RT-AC68U + 3G

I have the RT-AC68U and live in Thailand. I'll have to rely on 3G only for a month beginning next week, so I want to buy a 3G modem that will work with my router. The ASUS website only lists two compatible modems for the RT-AC68U and Thailand, namely Huawei E169 and E176. These are both old, I can't even find a shop that sells them here.

Could I use something newer like the Huawei E8231? Why wouldn't a 3G modem be compatible with the RT-AC68U?

Thanks