4G/LTE Internet for Vehicles
This is a complex question, and what’s ‘best’ will depend on the network, strength of signal, and whether it will penetrate the Faraday cage that your van effectively becomes.
Generally speaking, getting the external 4G/LTE antenna outside of the giant metal box is a Good Thing (TM), so you want something that’ll allow you to connect an external antenna. As Darren says above, the Teltonika range are good for this as they have SMA connections for both LTE and WiFi antennae, so you can use your own rather than being tied to the supplied units. Some of the higher end Huawei devices allow similar, but check carefully.
If you know you are in an area with strong 4G/LTE coverage, you may be able to get away with a mifi-style device, and this will likely be cheaper. But if you are in an area with marginal signal, an external antenna will likely help you achieve better speeds and more reliable connectivity.
We’ve ($dayjob) set up lots of mobile connections on portakabins for wildlife hides, roadworks, yard offices, etc. and our normal ‘go to’ kit is as follows:
(or RUT950 if you want to load balance with 2 SIMs or 2 networks): https://www.solwise.co.uk/4g-routers-rut950.html
Cross polarised external antenna: https://www.solwise.co.uk/4g-antenna-omni-xpol-a0001.html