The more gain an antenna has the more directional it is. A 0db antenna picks up equally in all directions, anything higher than 0 has some direction component. Your antenna are obviously very directional antennas that require pointing at specific towers.
A normal omni antenna is typically directional such that it picks up more of a disk instead of a sphere so its not good at above or below but has positive gain on the sides where the horizon typically is and where towers normally are. An omni can pickup multiple different towers in different directions.
You may be pointing those directionals at an overloaded tower or one they don’t want you on at the time, while the normal omni’s can get to a better tower maybe with lower signal strength but more backhaul or other factors. My dad use 2x MIMO directionals with ATT in a rural setting and he ends up having to repoint to different towers once in a while as one will go out or slow down and the other will be fine.
SNR is many times more important the raw signal strength, as mentioned the SNR of 0 is not good maybe your directionally picking up a noise source on that band better than the tower.
In my testing with both WiFi and my 8800L mixing antennas is fine as is doing only 2 out of 4. My 8800L seem to consistently perform better with my 2X external antenna than its 4x internals. I have my WiFi split 1x external 2x internal from my SOHO seems to be no downside while getting better external wifi. Typically transmit is not MIMO so one antenna will be doing upload would be nice to know which one. Ideally you have all 4 antennas as close to modem as possible (minimize cable run) with antennas as high as possible which is why domes with modems on roof with antennas work so well.
Amps amplify signal and noise and add their own noise, so not normally a good thing for receive / download unless they are close to antenna and you have a long run back to modem otherwise they can hurt receive. They can boost transmit though up to 1 watt but a Firstnet modem can do that too, not sure if Peplinks do that.