@jmmmm 

Hi

Fixed the MAC address of the device is not supported by routers. It is recommended that you set it on the device.
The USIM card is not much different from the ordinary SIM card, and the LTE router is compatible with it. It may be noted that the TL-MR6400 is a Nano SIM Card Slot.
For SIM card slot specifications,  you could search the product on the official website and click Spec to view it. For example:

https://www.tp-link.com/home-networking/3g-4g-router/tl-mr6400/#specifications

However, it is recommended that you purchase the router locally in Vietnam to ensure that it complies with the frequency band of the local internet service provider.