I finally replaced my old dying Sprint Netgear 6100D with a Cradlepoint COR IBR900. Although I had an SIM adapter and the 6100D's SIM card went in and was recognized, and I tried to register it with Sprint, the Sprint network wouldn't accept it (the Netgear 6100D SIMGLT207A won't work in the Cradlepoint IBR900). I had to go to a Sprint Corporate Sales office (Modesto, on Dale Road) to get the "right" SIM card (SIMGLW236C).
My connection to the new router is now excellent (i.e. my old 6100D was unpingable sometimes, but the IBR900 is always available). However for the connection to the Sprint network itself I'm only getting 7Mbps even though I'm in a LTE-Plus zone on Sprint's map. My RSSI is -71dB, RSRP is -100 dB, and SINR is 5.4 dB (I'm using a high gain external antenna). I know that the antenna is doing something because I get -71 with my high gain antenna and only -84 with the little included bunny ears. I've also got 2 of these high-gain antennas. But on Band 41, it still says that it isn't getting aggregation even though I've checked that both ports on the IBR900 are indeed active..
So I'm still envious of all of you who are getting tens of Mbps. I think that I've done everything that I can on my end.