I think commenter is suggesting a non custodial wallet with internet access would work fine, which is true. The problem seemed to be cell signal, not internet, as they were able to use the coinbase app or website.
Not disagreeing with any of you but a blockchain payment terminal that’s itself connected to the internet could technically receive your signed transaction from your (offline) phone and then submit it to the network on your behalf. The balance on your phone wouldn’t update until it itself gets back onto the network and resyncs its state with the blockchain.