Can I use my phone as a backup internet connection?

Yes, and for a very small business it is often the right first step. Know the limits before you rely on it:

• Tethering works for a few people - email, quoting, card payments. It struggles with a whole office.
• Check the plan’s data and speed caps - an outage lasting days can exhaust an allowance quickly.
• Same carrier is not real redundancy - if the outage is the mobile network, both paths fail together.
• Practise it once - find out now whether your gear tethers cleanly, not on the morning it matters.

A dedicated failover router with its own SIM is the next step up, and is no longer expensive.