I use my Ultra on WiFi. I have several different models of Ultras, including the latest 4850. They all will connect to WiFi at 200 Mbps or higher, with my 4850 hitting around 280 Mbps.
However, even though your phone might be capable of data rates that high, it requires a really good cellular signal to reach those speeds. If your phone is showing 1 or 2 bars at most, you'll never come close to those speeds. Assuming your Express is one of the older ones, it's a single band WiFi so can't reach the speeds available with dual band WiFi players. So any of the dual band Roku players should be capable of high enough connection speeds for MLB. But again, it's all dependent on the signal strength of your cellular signal.
bought an Ultra, brought it home, opened it up and immediately realized it's no good for me. It must only power up via usb and this one needs to be plugged into a 110v wall outlet.
Ah, I didn't realize you didn't have a 120v power source. Yeah, the Ultra won't run on USB power. But I believe every other version of Roku will, such as the Express 4K+, which is a very good performing player as well. Remember, your Express only supports 2.4GHz, which on Roku devices is far slower than a 5GHz connection. I would expect an Express 4K+ or any new version of the Stick would be just fine for your use.
We do have 110, but need to run a generator to get it in all the outlets. I bought a 12volt tv, so we could watch it without having to turn on the generator. Hopefully a 4K Roku Express or Stick will suffice. My old Express was not 4k
