Is it possible to "steal" 4G LTE bandwidth from an LB1120 modem?

My wireless (4G LTE) internet service has been disconnected several times without notice or reason provided.  

Until recently, I had been getting excellent bandwidth (AT&T Broadband) but then, suddenly, nothing! 

I tried switching to T-Mobile SIMs (twice).  They worked fine for one day and them throttled me back to 600Kbps for the next 30 days.  Went back to AT&T SIM and got excellent speeds for just over a week but then "poof" - all gone, again!

My Netgear router traffic meter showed I normally do less than 150GB/mo. (avg. 3800MB/day) - well within normal caps, I'm told.  We don't do gaming, no 4K TVs, nothing that is high bandwidth, as far as I can tell.

So, I 'm trying to determine what could be causing the spikes that would cause AT&T or T-Mobile to chop me.  Does anybody have any suggestions how to trace these spikes - could someone be "piggy-backing" my service?

LB1120:  latest firmware, Bridge mode, ethernet connect to Orbi AX4200 Mesh routing system.