Hi to everyone,
I have GL-MT300N-V2 - firmware 3.025
BusyBox v1.28.3 - OpenWrt 18.06.1
I solved this issue by editing the file tertfinfo_bak and rebooting the device:
DISCLAIMER: If you are not fully confident in what you are doing please dont do go further!!!
- login in into ssh shell
- edit /etc/tertf/tertfinfo_bak
- delete offline device lines from the file
- save the file
- reboot the device
Enjoy.
[edit] other info in this post: How to remove entries in the client list? - #7 by James13