@TP-Link Deco I found the problem, i forgot the cable from modem to switch so i had a network loop. I remove it an now everything is fine (about 680Mbps on booth Deco modules)

@TP-Link Deco Hello i have a strange problem with my Backhaul conection between 2 Deco M5 devices. If i connect them like this: Then my Modem/Router (SpeedPort Plus) is restarting every 4-5 minutes....