Instead of paying for a separate SIM, I've been piggybacking on the "data only" SIMs that Google Fi let's you add to a normal subscription. No extra per-SIM fees, and you just pay for whatever data it uses at $10/gb.
More details on my modem, monitoring setup, and IPMI backhaul strategy using Wireguard here: http://jsharkey.org/ipmi/
More details on my modem, monitoring setup, and IPMI backhaul strategy using Wireguard here: http://jsharkey.org/ipmi/