@Kevin_Z I have resolved the issue by a 'WDS Bridge' from the MAC address of the Wired LAN to the Wireless 2.4Ghz Setting. Somehow, this now works flawlessly, just like the C8 does without this fix....

Dear TP-Link Community, I am having trouble establishing an Internet Connection through my TP-Link Archer C9 in the following set-up: ISP Modem [LAN Port - 192.168.0.1] - 24-Port Switch TL-SG3424...