Build a terraced end house with an additional unit (GÜ) on your own

If without a basement and HAR in the attic: Then hang an access point in the hallway on the ground floor and put the FritzBox in the HAR (you can always connect an access point if needed). For the upper floor, I would initially just lay an empty conduit in the hallway there, and if necessary, a second or third access point can be added there. Depending on the reception, you will then see if another access point is even worthwhile.

Unfortunately, iPhones no longer have a direct network connection,

The wives I know do not watch their Rosamunde Pilcher on tablets or even smaller screens.

If the appearance is disturbing, make a box without insert, coil the LAN cable inside, and cover it with a blank cover taped over.

If the appearance bothers you, make a box without an insert, coil the LAN cable inside it and cover it with a blank cover.

I'm concerned about the appearance of the APs: then I would have a ceiling lamp, access point, and smoke detector on the ceiling in the small hallway. Not exactly beautiful.

Don't you have a niche in the hallway where a floor-to-ceiling built-in closet would fit? Behind it on the wall, a flush-mounted box with the cable, place the access point itself in the closet (or if necessary, on top). A lamp itself can be a decorative element that shapes the room, and the smoke detector, well, just a technical necessity...

Don't you have a niche in the hallway where a floor-to-ceiling built-in wardrobe would fit? Behind it on the wall, an in-wall box with the cable, place the access point itself inside the wardrobe (or if necessary, on top of it).

On the ground floor we have a coat rack. That would work. But not on the upper floor and attic - it's an end-terrace house. The hallways there are very spacious.