I have the same problem as you and my shareplay refuses to work, I even bought the same modem and router you did and still nothing. My speeds are faster but shareplay still refuses to work. My friend was able to connect once and then never again after that. This is really frustrating.
If I still couldn't get it going after buying the new devices, my next move was going to be to replace all Ethernet cables involved. I'm sure I've closed a door on them once or twice. Or more. And sometimes they go bad for no reason. Even brand new ones. It couldn't hurt to try. Also, if you haven't yet contacted Sony tech support, I would do that as well to be absolutely sure your PS4 settings are all correct. Good luck!
