MiFis often have a low limit on the number of devices that can connect to it, which may be why one device works and another doesn't. Amtrak has sometimes issued MiFis, especially to sleeper car attendants, for use in sleeper cars to provide a makeshift wi-fi network where the train itself doesn't have the equipment to provide the official Amtrak wi-fi.
Amtrak's on-board wi-fi performance is generally hit-or-miss, even with the official equipment. Sometimes it works well, especially on the NEC or where cell signal from multiple providers is good. In more rural areas, the service tends to be slow and spotty. Outside of the NEC, Amtrak relies exclusively (as far as I'm aware) on cell towers for internet access, and in rural areas there isn't many cell towers to share the load between. They also have to share bandwidth with cellular users in the area (both on and off the train) so it winds up being a small-ish amount of bandwidth shared among a couple hundred devices if the train is busy. It's better than nothing, especially if you have Sprint, T-Mobile (though less so these days,) or a regional carrier, but it's not as good as even most coffee shop or hotel wi-fi.