ZTE MF258K PLAY Freezes, Signal Interruption & Speed Fluctuations - Antenna Direction Issues?

@ empe6 :
with such high signal levels from the antenna you could have larger SINR and RSRQ, but you can see this location, you have interference around.

As for the location of the antenna on the mast, it looks from the photo that the MF is too close to this piece of aluminum, which is the DVB-T antenna. This may worsen the performance of LTE signals a bit, but this should not be the cause of these drastic speed jumps / lack of net. Rather, it could show up interference on DVB-T.

The distance between the antennas should be> 0.5m, but this is not the place for you, because lowering the MF means getting closer to the barrier, which could deteriorate the signal. You can try to fix the antenna position by setting it to the max SINR for individual bands, but you have very little space, so it's rather hard to suppose that you would feel any significant difference.
If anything, then maybe a separate mount for the MF, but only if you would get a SINR> 20dB, so that you could jump on the QAM256, if such a station is transmitting on one of the bands.

Try to check the parameters of the signals at the moment when the transfer is falling - especially the SINR value. But, I have a feeling that the station is the cause.

QAM256 jumped in, so there is a lot to fight for with the antenna, because it is 30% faster than QAM64 "for free".

It's not good that the modem behaves so strangely. It is true that BTS decides to enable aggregation, but it's a bit suspicious. There may be a modem failure in such a system.

Do you have the possibility to check the sim on the phone with aggregation?

Such jumps could result in jumping over BTSs or sectors from a given BTS.

Signal levels and SINR are high - did you try to move the antenna inside?