This article discusses USB Restricted Mode in iOS and PayPoint Devices and the resolution

Please see the information below to correct the iPad not communicating with the PayPoint. 

Executive Summary
A security feature introduced in iOS, USB Restricted Mode, may introduce a scenario in which iPad is unable to communicate with the PayPoint dock after being restarted or if the device is locked and in standby for 60 minutes or more. Symptoms may include iOS apps being unable to communicate with PayPoint’s magnetic stripe reader, barcode scanner and customer-facing display.

Affected Elo Devices
PayPoint® for iPad®
PayPoint® Plus for iPad®

Affected iOS Versions
iOS 11.4.1 and above

Resolution
To avoid this scenario, it is advised that users make a settings adjustment located in iOS Settings Touch ID & Passcode USB Accessories.

By default, this setting is switched to disabled. When switched to enabled (GREEN), USB communication will continue to work as it did before the release of iOS 11.4.1. This will allow iOS to communicate with the PayPoint dock unrestricted.