If C20i works as the main router, you should confirm if the old router has WDS function or not and you should configure it on the old router.

codoardi wrote

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Hardware Version : Not Clear

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ISP :

Hi, I own an AC750 and I connected my ISP cable modem to it in my basement. The signal strength is weak upstairs so I'd like to use my old wireless router in a bridge configuration for upstairs. My question is: Can my TP-LINK AC750 Archer C20i act as a bridge so PCs upstairs can wirelessly link to it & access the internet like downstairs PCs can?

Please help.

Yes, you can use this router as a bridge. Before doing anything further find out the IP address of your old router which you are trying to bridge. Login to C20i interface and first change the router IP address in a way that first three dot separated values are same as old router. You can use any final number. For example: if IP address of your old router is 192.168.1.1, you should make 192.168.1.XXX where XXX can be any number, say 10.

Next, you have to enable WDS under 2.4 or 5 GHz (not both) wireless connection of Archer C20i interface, scan for your existing wifi SSID and connect to it using existing credentials of the SSID. Then go to DHCP and disable it.

Do a reboot and you are good to go.