Hello,
Remote access to the router has to be first enabled in the router's WebUI System -> Administration -> Access control section. Besides, most of the routers functionality is disabled, when the router operates in passthrough mode, including its WebUI access.
Also, more details, related to your topology and configuration settings would be necessary to evaluate the issue.
Have you checked this Ubiquiti forum thread related to your issue?
Best regards,
Žygimantas