It looks like the Mudi device works with T-Mobile, and I saw some posts about getting it to work with AT&T, but does it work with Sprint or any other carriers also (USA)?
Also saw a couple lesser-known companies selling unlimited data sims, would those work with this?
Also, people talking about Mudi in other forum threads are talking about getting it to work on last generation network technology (LTE).
Does GL.iNet have a 5G SIM router product in the works?
There aren’t many 5G mPCIe cards available on the market - until that happens, it’s quite unlikely that there will be a 5G Mudi.
The telephony part of the device is a swappable (not by the user, sadly, at least not easily) mPCIe card, which is why it’s so simple to support multiple markets.
Carrier support is based on the frequency bands used. In the US if I’m not mistaken, there are 4 carriers (well, now only three with the Tmo-Sprint merger) that have actual physical networks, the others are just piggybacking on these (the so called MVNOs or virtual mobile operators). You need to check the area you’re in for carriers and their specific bands, and then decide on the Mudi you need. The available cards are these for Cat6 which is faster, but more expensive, and these are for Cat4.
Given how the Cat4 modules are broken down into smaller regions, I’d recommend you go for the Cat6 North America one, however make sure that the carriers in your area support the bands listed!
Other than that, all you need is a nanoSIM card from the carrier, and you have internet.
Sprint & Verizon are not good choice. Try AT&T and T-Mobile.
We do not have 5G products now. But we will.
Please first release a new variant of the Mudi with a more accessible mPCIe card slot 
I know you can only guarantee functionality with cards you’ve specifically tested, but I see no reason to not be able to swap to a card that uses the same protocol.