Tenant moved out early from lease but not responding

[Vanessa Leeper Jones]

Hi everyone, I have a tenant who is 9 months into their 1 year lease and has been asking about options for moving out early from the lease due to them expanding their business into a different state. The lease stated a 1 month early termination fee, so I let them know we would need to receive the early termination fee first and their decision to terminate early in writing if they wanted to terminate early. I've followed up for 3 weeks (phone, email, text) asking about their what their decision was, and it's been 10 days since they stopped their utilities, and the neighbors said they moved out. Since I don't have written confirmation that they want to terminate early (and don't have the lease breaking fee), how should I proceed with them not responding? In the next few days October rent will be due and if they don't pay rent, they've moved out according to neighbors, and they aren't responding, can I give them an ultimatum to respond/pay rent and if they don't the lease will be considered terminated since they moved out, haven't payed the latest month's rent, and aren't paying utilities? thank you everyone for your help!