Yes but according to previous posts, the modem will be near the entry point. The existing master socket is NOT near the entry point, hence the existing Cat 6 run from the master socket to the under stairs cupboard won't be useful.
With ours I laid some conduit from where the cable emerges from underground across the front of the house under some chippings. The engineer was able to then drill through the wall to mount the ONT in a better place. They might be able to simply run the fibre cable/clip it to the wall to run it to where is best for you?