Although unusual in a domestic setting , and more common in a business setting , if the duct from outside actually enters the building, it needs to have a duct seal , not just to stop any potential ‘gas’ in the underground network entering the property, ( an explosive risk) but also should the underground network flood with water , that won’t enter the building via the duct either.
TBH , posting here on a BT forum is pointless , this has nothing to do with BT , as far as Openreach ( presumably this company have tried to use OR ducts ) because it’s not Openreach installing the service ,they quite correctly will have no interest dealing with your issues with City Fibre , so if this competitor company doesn’t have the ways or means to re-seal the duct after installing their cable , then presumably that’s it , it’s not BT or OR ‘fault’ that this company that is allowed a parasitic relationship with OR by leeching on OR ducts , don’t appear to have the necessary solution to this issue, which as stated is unusual but not unique, especially since they also do business installs where ducts inside premises is very common.
I would suggest that this Alt Net need to employ whatever process they would use if your property didn’t have a duct at all , or if the existing duct were blocked inside your curtilage, by installing their own duct from the footpath to the external wall of your property, but if they only do ‘cheap’ installations that don’t require them to pay for extra work , they may simply decide not to provide to you with service, and walk away , if that’s the case that is between you and them and nothing at all to do with BT.
