Best way to stick a very long Ethernet cable semi-permanently to walls?
The problem with almost any adhesive is that the longer it stays on, the more likely it is to either fall off when you don't want it to fall off, or stay on when you don't want it to (typically this means &quot;top layer of paint comes off with the tape&quot;). Aesthetic considerations may not be an issue right now (when I was a kid, I definitely ran wires around in unattractive ways, though not Ethernet, that wasn't an option at the time), but a &quot;temporary&quot; wire hanging around is less troublesome than peeled patches of paint.
My recommendation is cable clips/staples. There are plenty of varieties, but generally something like this:

There are some with a single nail and some with two nails. For Ethernet (and similar), a single nail is strong enough. If the main part is white and you switched to a white Ethernet cable then the whole thing would come pretty close to &quot;background noise&quot; on a white wall. But if you don't care about the look, stick with the existing cable, it is just fine.
I definitely advise against switching to WiFi as others have suggested. Nothing beats a wired connection.