Critics say the incident underlines the vulnerability of a banking system that relies on vans rather than bricks-and-mortar branches.
Staff on the RBS mobile bank were unable to provide printed bank statements or other information when the signal failed in Turriff, Aberdeenshire.
RBS closed its Turriff branch last month, after announcing 62 closures in Scotland and a further 162 in England and Wales.
One customer, Marj Chalmers, chairwoman of Turriff Business Association, said it was the final straw and she now plans to move the group’s account elsewhere. She said: “I was in and asked for the details, but they could only give me the information on a note.”
Former RBS worker and now local councillor, Alastair Forsyth, said poor internet signal was one of the “vulnerabilities” of the mobile service.
RBS confirmed their satellite technology was not working and they reverted to “contingency methods”.
In at least one case, staff had to scribble the information down and hand it over to customers on a slip of paper.
Mrs Chalmers, said: “I did ask for information about the business association account, but they just put it on a Post-it note rather than a compliments slip.
“I was in and asked for the details, but they could only give me the information on a note.
“The mobile service is all right, but you are standing outside waiting to go in so it might not be so good in bad weather. I did hear staff tell one person the internet signal was going on and off. I’m closing our account with RBS and moving to the Bank of Scotland.”
Councillor Alastair Forsyth said: “The disappointment and inconvenience experienced by customers in the circumstances of failure to connect to their server rendering the service inoperable cannot be overstated.
“I trust that RBS will be monitoring the service with a view to improvement.”
A Royal Bank of Scotland spokesman said: “I can confirm that the satellite technology was unfortunately not working when the mobile branch visited Turriff, however staff on the branch did continue to serve customers using contingency methods.”