There are probably cheaper options, especially if you want to literally use your smartphone as the hotspot, but the Verizon home backup internet plan I looked at recently is $20 a month (and gives you 7 24-hour periods per month of unlimited data).

The Verizon home router is included when you sign up (you have to return it if you cancel). I bought it out of desperation when my home fiber internet was down all weekend due to a local tech screwing up and unplugging my house from the breakout box on the curb, but given the cost and the normal reliability of my home internet it’s really barely worth it.