If a city has an extended power outage such that the battery backed cell network goes down, then everything else will be failing too and payments are the least of your problems.

Without electricity the water system depressurized, which contaminates it. After about a week the sewage pumping stations have backed up so the sewer system is starting to fail.

Modern cities cannot operate without electrical power given their scale and density.

It is bizarre to think the biggest problem is "how do we keep a transaction of value?"

Like, just declare an emergency and let business owners be reimbursed by the government.