TBH , I think this is a misunderstanding with what ‘BT’ are assuming Openreach can and will do in what is a pretty unusual situation,
If the master phone socket ‘works’ , in other words if you plugged a standard corded telephone into it , and you could make and receive calls , then as far as Openreach are concerned, there isn’t a fault , if the property doesn’t currently have electricity , so no point in connecting the router , is not your provider (BT) or OR’s ‘problem’ , if you made this clear to ‘BT’ , then BT raising a fault with Openreach was always the incorrect approach , and OR cancelling the ‘fault’ visit is understandable, OR will remotely test the line and see it ‘tests’ OK , any ‘notes’ about house fires etc would not be acted upon.
A separate issue is , if your request to BT was to maintain broadband at the temporary accommodation during the power outage at the ‘main’ address , then offering the mini hub etc. ( assuming the alternative accommodation has power ) does gets you back on line , you may feel that this is substandard and that paying ‘full price’ for less than a full service is wrong , but the alternative would be to ( as you may initially wanted to have done ) was to cancel you service completely, and if still within a minimum term contract paying early termination charges, until the property is habitable again , then reorder with a new minimum term contract etc.
FWIW , you probably should ( if you feel the cost is excessive for the service provided and there are no onerous penalty’s ) is go back to BT and cancel , the details of the fire are probably only going to confuse the matter, simply say you want to cancel, send back the mini hub and any other equipment when requested, and make other ‘temporary’ arrangements at the mobile home …when the house is habitable again , order new service with whoever you want to use.
If the existing setup , although not ideal, is acceptable, continue as you are without expecting an OR visit to rearrange landline service from the house to mobile home , after all if they did move service to the mobile home ( assuming that’s even possible ) this would have to be reversed at some point in the future, so OR have the expense of this work , when it isn’t Openreach responsibility as it isn’t a fault in the recognised sense.
As already stated , if you have mains power at the temporary accommodation, you could either run a power extension to the house for the normal router , and hopefully pick up WiFi in the mobile home, or provide a phone extension lead from the phone socket in the house , into the mobile home and relocate the normal router to the mobile home getting you back on line using the ‘landline’ service, cancel the ‘fault’ and send back the 4g mini hub, doing either of these things is something you would need to organise yourself, this isn’t really something BT should be suggesting OR will do for you as part of a fault report , because it isn’t, but as indicated, you could ask for an external extension to be provided from the house to mobile home , a service you would be charged for by BT , but it may be cheaper/quicker getting it organised independently
