You don't, it declines and you agree to go back if possible in an area where you know the signal is better.
Or in my case 9 years ago, getting kicked off the train. I had no means to buy an anytime return from Hinckley to Atherstone at 06:20 at the time (ticket office opened after the train I needed at 6:34am, and i was forced to go that early as there were only 4 trains a day that stopped at Atherstone in the relevant direction) so I was forced to buy off the train. I had a Basic Bank Account with Lloyds with no offline auth, so when the machine lost signal close to Nuneaton, it would decline and there wouldn't usually be time to re-try the transaction before I got off, leaving the guard in an awkward position as if he kicked me off the train there was little he could do. This would repeat ad infinitum 5 days a week until I moved back to the South East
.
I usually had around 10mins for my transfer at Nuneaton, so I'd go into the ticket office at Nuneaton where my card would be accepted.
But it got frustrating that both the guard didn't understand that I had means to pay, and that my bank wouldn't acknowledge the existence of offline auth on their cards. I just kept to having cash for the fare to hand in the absence of a genuine answer, and eventually bought a bike when I'd saved up.