@Valux 
Do the signal parameters (RSRP/RSRQ/SNR) improve when you connect the external antenna? Maybe you can post the before and after values.

How long is the antenna cable and what type of cable is it?

Chances are that the cell is overloaded. And the Iphone has most likely a better LTE modem than the MR100.

  @kernash The first image is with the default antenas provided with the router and the second one is with one lpda antena directly connected to MAIN and original antena on AUX connected. Yesterday i moved the antena a little side to side to try and managed to get a RSRQ: -7dB and SNR: 20dB but thats the best i got and still all i could achieve on speedtest was 40 mbps maximum

.

  @Valux 
The signal is a lot better on the second image.
So the Iphone is probably using different bands than the MR100. I don't know if there are any apps for the iphone to see which bands are used. Maybe you can check that.

  @kernash 

I've checked it and iphone is using only bands 1,3 and 20 so i put the same on the router for now and will try. I've noticed that internet is always really slow from 6pm to 12 pm. I assume is due to high usage of the tower, sometimes i have even only 2 or 3 mbps download at around 8 pm, is there a turn around for that? 

  @Valux 
MR100 is a LTE category 4 router, which means it can only use one band at a time, while your phone can use all 3 bands simultaneously. So it will never be as fast as your phone. You can try to set the bands manually and see if one band is faster than the others.

And yes, if the cell is busy, there is not much you can do.