LM1200 Signal Optimization

Using an ATT SIM in a rural setting. ATT tower approx 4 miles away. Have used the LM1200 for 6 months with no external antenna with acceptable results. Recently performance began to degrade for whatever reason. Purchased external MIMO omni antenna. Bars went from 0-3 to 3-5. RSRP and RSRQ improved 25-30% (were -112/-13). Performance is back to what it was 20 dn & 7 up. RS-SINR remains at 0 - QUESTION = is that always the case? I would think that knowing about noise/interference might be helpful in antenna placement. Any tips on placing the omni? I have trees that I cannot get above, but there are no hills blocking my view of the tower. Also the bands switched with the new antenna. Mostly B66 before and B14 now. Any advice would be appreciated.