I got one mAP 2n for test and connected USB WIMAX Modem.

It looks like USB conector not capable to provide enough current to modem and it goes to reboot.
We've tried POE connection and direct connection with standart power supply 24v x 0,8A . There is no big difference. With direct power restarts is less often.

Anyone succeded to run high power USB 3G/4G modems with this device?

USB power will not change based on PoE voltage. USB port is not high-amperage, so power hungry devices may not work.

I am just worry why Mikrotik has no good and cheap boards for making 4g Boxes with USB dongles?

We are using RB750UP - this is the oldest and the chipest gear for this. We have to make stupid unboxing of every device to extract board.

It would be much better to use some kind of 711/911/912 low end version. But for strange reason only top models has USB with up to 3 time higher price.

map 2n is disappointment.