Model : TL-R600VPN

Hardware Version :

Firmware Version : 1.2.3 Build 140801 Rel.49374n

ISP : SFR

Hello,
My router is behind a cable modem (192.168.10.x), the router manage a network with 20 clients (wifi connection through a netgear WNDR4300 IP 192.168.0.250 + ethernet clients).
The DHCP allow 50 addresses (192.168.0.100 to 150), I have some IP reservations (3 on the DHCP range and 5 on static range between 192.168.0.10 and 192.168.0.50).
The issue is on a particular Lan port #4 link with a switch not manageable (Dlink GO-SW-8G), this part of network is a 100MB, the other (direct link with the router) is a 1GB.
The VPM is disabled.
Every day this port are off. The only solution is to unplug the cable on port 4 during 10 seconds and replug. Sometime I need to reboot also the router.
This issue appears since 3 weeks, without change on the setup and on the network. I changed the switch, the cable, the port (from 4 to 2) without improvement.
During the issue the other port 1,2 & 3 works well.

Any ideas ?

Thank you for your input.