Hello,
How do I ground RB951G-2HnD router? I can see that its ports have metal frames, but its power supply has no ground. Apparently, it has "floating" ground (pls, forgive me if I am mixing terms), but I could not find anywhere else where I could connect ground externally.
Or does RB951G-2HnD not support grounding at all?

AC/DC power supplies have internal isolation so there is no "ground" on DC side.
As RJ45 sockets have metal outer frames then it is possible make a connection between 951 and "the other side" via this frame but you need to use FTP or STP or SFTP cable to connect these frames.
If you use standard UTP cable then "the loop" is made with pins, not the metal frame.
BTW ... grounding should be done only on one side of S/FTP cable AFAIK.