More details on my modem, monitoring setup, and IPMI backhaul strategy using Wireguard here: http://jsharkey.org/ipmi/
I can pay my mobile provider an extra £7.50 per month for a "second SIM" to share the allowance from my main SIM (you're effectively paying to rent another number).
It's not a terrible expense, but for something I need to send an SMS maybe 1 per quarter is a little much. I guess if you look at it as a "service" it's not bad, but this was a home thing, nothing critical would be lost if I couldn't get my home IP until I got home. Now I use DDNS using a small piece of Go code I wrote to update the domain on my Digital Ocean account as a form of "free" DDNS, it runs on my router on connect/reboot.