I have an ATT prepaid tablet/hotspot plan. I've used virtually zero of the 15GB of data. When I put the SIM in a tablet, I get about 30-40 Mbps via speedtest.net. If I take the SIM out and put it in my (R11e-LTE-US, which is listed as being a supported device) Mikrotik Hotspot, I get < 5Mbps. This is with the same APN details that the tablet is using, and at the exact same location. The tablet shows about -110 dBm, whereas the router shows -71dBm.

So, it's certainly not the time of day (every time I've tested it, I tested at the same time as the tablet), and I've certainly not run out of data (I have 14.5GB remaining).

I've tried updating to ROS 7.4, then downgrading. The only settings I've played with have been APN.

Any ideas what the deal is?

Some ISPs are known to assign a bandwidth limiter according to the TTL value, which normally decreases each time a packet is routed, in order to prevent sharing the connection, but since your plan is a tablet/hotspot one, it is unlikely to be the case here.

Other ISPs have a database of IMEI prefixes; with these, if they declare Mikrotik to be a supported device, I'd start by checking with them whether the prefix of your modem's IMEI is in the database (it may be from a new series they haven't added to the database yet).

There is also a possibility that the tablet and the Mikrotik get pointed to a different LTE band (it's the network to determine which band(s) to permit to the MS); if your tablet is an Android one, use Network Cell Info Lite to check this assumption.

Other ISPs have a database of IMEI prefixes; with these, if they declare Mikrotik to be a supported device, I'd start by checking with them whether the prefix of your modem's IMEI is in the database (it may be from a new series they haven't added to the database yet).

There is also a possibility that the tablet and the Mikrotik get pointed to a different LTE band (it's the network to determine which band(s) to permit to the MS); if your tablet is an Android one, use Network Cell Info Lite to check this assumption.

Ironically, when I entered my IMEI into the AT&T IMEI checker, it tells me it's a Moxee mobile hotspot (maybe the Moxee has an R11E-LTE-US PCI card?)

Yes, the mini PCie contains the IMEI, so "it's" the Moxee mobile hotspot.

Regards.