With a little bit of budget, you can get Huawei B525 with 600 Mbps LTE performance around RM500.
Most modems that around that budget you give is not LTE-A, so you cannot take full advantage of the speed to the max.
If you do want that Huawei B525, I can give you the link.

Many people said now there is less B618 because Huawei is replacing it with B818.
But maybe TM did not want it because it is too expensive.
So they replace it with a subpar modem which is ZTE MF286C.

For LTE-A modem the budget is a bit high than a typical modem.
Plus with unifi Air using a different band than the three top telcos, so your budget will be a bit high.
I think the cheapest is B525 which is around 400+.
But of course the best is B618-65d which can pull more signal and better stability in terms of speed.

Yeap but not the B618.
I am using it with B818.
Well you pay cheap, get slower speeds.
But of course that is also depends on how crowded the tower is in that area.

Well if you got fibre, why not stick to that?
If I do have the choice to subscribe to fibre, I will subscribe to it.
Lower ping and more stable speeds.
The only issue is I am left to choose of Streamyx 4 Mbps or unifi Air.
As long unifi Air is able to give good download speeds, I am satisfied already.