i too was having this same issue, which i was able to fix by giving KDE Connect on Android all required permission and installing sshfs (from octopi or CLI whichever is your preferred way)
sshfs installed because if you check the info about kdeconnect on octopi there it mentions the use of sshfs as optional dependencies for remote filesystem browser, upon installation i was able to browse my android device i Dolphin File Manager when both pc/laptop and mobile are connected to same wifi
and one last thing if you’re unable to add device check this Kdeconnect not connecting - #15 by vawaver