My rented apartment has no ethernet cable runs between the rooms (even though the building was only built in 2018), only coax.

After the third MoCA adapter dying within 5 years and with neither WiFi mesh nor powerline cutting it for me I was looking for another solution.

Enter "invisible" bend insensitive fiber (G.657.A2 / G.657.B3).

It's under a millimeter in diameter and basically vanishes into corners and base board crevices. From more than a meter away is't completely unnoticeable.

Together with a pair of bidirectional SFP transceivers this makes an amazing retrofit option for locations where laying new runs is not an option.