Anyway guys finally my Huawei B818 router has finally arrived.
Seriously I am impressed with the modem.
Can get stable and solid signal throughout.
Really worth the investment for my Unifi Air SIM only plan.
Thanks to ashdwoers for advising me not to take the ZTE modem contract.

This is my speed I get with my Unifi Air + Huawei B818:

Average is 20 Mbps.
But it is able to go much higher if the line there is not crowded.
Connected to Band 40 with 2CA.
In other words, LTE-A.
I think it is the same SIM like everyone get.