Setting up a wireless bridge on Cisco DPC3825

I'm trying to expand the weak signal of my Cisco DPC3825 modem/router with a TP-Link TL-WR841N  router. I can sort of get it to work, but all my devices keep switching back and forth once I have it set up. I found a setting on the DPC3825 under Wireless -> WDS Settings that says "Allow wireless signal to be repeated by a repeater", but I can't select it because there is a big red message that reads:

"Note: Only WEP encryption mode or encryption disabled can be used with WDS. Before enabling WDS, please set the "Wireless Configuration" to "Manual" and the "Wireless Security Mode" to "WEP" on the Wireless Security page."

I can't (and don't want to) enable WEP as the encryption, but it appears that is required to enable bridging?

There are then 3 fields that I can input a MAC address of the bridges I am using, but it won't save anything I put in there. I assume that is because I can't check the above setting.

Help is greatly appreciated.