Minecraft version: 1.12.2
CC: Tweaked version: 1.80pr1.11

Peripherals connected via cables and modem aren't listed in a peripherals.getNames() call.
They also can't be wrapped with peripheral.wrap.

Additionally, when wrapping a peripheral by the side they are on ( which works ) every call just returns the error: "lua:1: Not attached to this computer"
Although this may be an issue with plethora, I don't know

Finally, calling any methods except for getDocs via a modem and callRemote returns "unsupported operation"

I could reproduce it with:
Minecraft 1.12.2
Forge 14.23.5.27.95
CC:Tweaked 1.80.pr1.11
PeripheralsPlusOne 1.12.2-build-T58
Plethora 1.12.2-1.1.13
TheFramework 1.12.2-1.0.1-build-T23

This seems to be fixed with newer versions of PeripheralsPlusOne.
Sorry for bothering you

Ahhh, no worries. Peripherals+1 does some rather weird things, which has definitely caused issues in the past. I think they're fixed mostly fixed in the latest builds though.