REIN, if there is that on your line, doesn't fall into the standard service by Openreach. As Black Sheep puts it on here: it's a gratis service, also implying that one should be very grateful to Openreach if they investigate it. Regardless, they're not obliged to investigate anything related to REIN, if it is indeed on your line, as you've not demonstrated that yet.
From my experience, and to be clear it's only mine, getting Openreach to fix things can sometimes be very difficult.