The flat's access point would be ethernet, unless you somehow want to convert the lan cable you plan on pulling to there to BNC?
lol
The problem with PPPoE is you lose the ability to have access to the internet wirelessly, so any wireless device (like your phone) needs to be capable of dialing up to the internet as well, before you access the internet. Huge PITA.
You also lose some "router" capability as far as I can remember.
Even though it might sound like a good idea, because they will use their bandwidth, the bandwidth of the line (4mbps) stay's the same and there isn't a 50/50 split on it at all. So if they decide to torrent, it will still eat up all available connections/bandwidth even if using their own ADSL accounts.
This can be very ****ty if you expect to game on your ADSL at all, stream a movie or anything for that matter.
What you could do is grab a Mikrotik and set it up to do some basic shaping/throttling giving your house and the flat and even split in bandwidth usage. Priortize streaming/http/gaming for example vs torrents etc.
Much easier to do that and pretty much will ensure fair use on both parties case.
You could take it a step further, setup a small media server, have a seedbox somewhere (mine is in France) and have something like a shared Sonarr install on the media server monitor for TV shows you both (or just one person) watches. As soon as a show is available, the download kicks off on the seedbox, and an http/sftp connection from your end to the media server happens, so no torrents on your own network.
Then both of you can enjoy the TV shows off of the media center instead of re-downloading the same episode twice (as an example)
Also consider an upgrade from 4mbps to something higher
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