I’m posting this here in case this helps anyone else who is using AT&T. 2 times now during Peplink software upgrades, AT&T has decided to put me on some sort of degraded cell tower plan/QOS bucket. I’m using the 100GB Prepaid $55 plan in my device (Max Transit Cat 18) which gets 80-160Mbps down and 20-28Mbps up. During my RC3 upgrade and my RC6 upgrade on 8.3 I somehow ended up being classified on the tower as something else which limited my download bandwidth to 8-12Mbps. Upload bandwidth was unaffected. I also noticed that my SpeedFusion tunnel on this connection was seeing packet loss in that download direction. So it seems like I was being put into a very low class of service / QOS queue by the cell tower. I tried B5, B66, B2, B30, B12, B14 and none of the bands had any impact, it was definitely a tower defined restriction and not something the modem or airwaves were doing. The first time it happened I troubleshot it for 1 day before fixing and the second time I spent 3 days trying things before I found the trick.
Long story short, changing the modem to “generic” firmware and then back to “AT&T” Firmware returns the service to normal. I don’t know what it is that the cell tower is seeing and doesn’t like, but at least there is an easy fix to get AT&T to start seeing you as “one of their own” again. (and giving you the higher priority queue on the tower)