Question about TP-Link M7350 Mobile 4G/LTE WLAN router

Great, that means it will work perfectly in Vietnam:
Viettel’s Mobile Network Bands
Technology Bands / Frequencies
4G / LTE B1 (2100 MHz), B3 (1800 MHz), B8 (900 MHz)
In addition, some sources list B7 (2600 MHz) (used in dense urban areas).
3G / UMTS / WCDMA B1 (2100 MHz) and B2 (1900 MHz)
2G / GSM / EDGE B3 (1800 MHz) and B8 (900 MHz)
Thanks.

Yes, this model supports LTE bands 1, 3, and 8. Specifically, it supports 4G LTE FDD bands 1 (2100 MHz), 3 (1800 MHz), 7 (2600 MHz), 8 (900 MHz), and 20 (800 MHz).