Should you request a payment using Pay by Bank App, where for some reason your customer doesn't open their banking app within the set time frame of 4 minutes, all you have to do is go back and request a new payment.
The first payment request will simply be cancelled and your customer will not be able to make a payment. Once you request a new payment, the customer will receive a fresh link with a renewed time period of 4 minutes within which to complete the payment. There is a timer on the page so you can monitor accordingly.
In the event there are any further issues for the customer with opening the link on their text message then you can supply them with the 6 Letter Code (e.g. BGZTCP in example above) that appears on the payment screen, that they can input directly into the Pay by Bank app section of their mobile banking app.