I’m successfully using a mofinetwork.com router with a t-mobile data plan. The router is rather pricey, but I needed something more robust than a consumer grade hotspot for use in a very hot garage. To get around the NAT issue, I utilize mofinetwork’s option to provide a static IP through their cloud servers for an additional monthly fee. This allows me to access the OpenSprinkler directly through the internet. No special routing was needed other than a port forward in the router. The router looks to be capable of running other VPN clients which may provide a static IP as well, but I haven’t explored that option.