Alternative SIM / Data only providers

Lets face it, the SixFab SIM is not really a great bargain if you are needing to transfer large amounts of data.
Who are you using for your provider for your data?
I am working towards setting up the SixFab hat as a backup link to my main (cable) internet provider.
My router has a secondary WAN port that should “fail over” if the primary link goes down.
I may need to look into allowing just “necessary” traffic through that backup link so as to not drain my bank account during an outage of my main provider.