Disconnections, Reboots

At first I was excited by the router - uncluttered Web interface, fast, good functionality.
He is now almost 1 year in use since 1/2 year, the problems multiply - new devices in the WLAN will not be accepted, canceled existing, the router will reboot (then everything works again for a certain time). Meanwhile, the reboot is performed approximately in 2-
Day rhythm.

I use the device in the office (only WLAN + routing to another network) - ca.5 lasting + up to 10 temporary units. But definitely not suitable! (VPN and SSH connections constantly break down)
Have always the latest FW recorded, nothing has brought. Unfortunately, that is also complex since all settings are reset.