You'd better post a brief topology of your network. If TL-WA701ND is working as an access point, it should be connected to root device with a network cable to access internet and broadcast its SSID....

I think 830RE and Fritsbox are using IP address, 192.168.0.254. Turn off your Fritsbox and try again. If you can login, change default IP of 830RE to 192.168.0.250. Power on your router.

I don't think TL-WDR4300 can WDS to TD-W8970 successfully because of chipset limitation.

Try different channels and change channel width to 20MHz on wireless settings page. Check status of WLAN light. If it go off constantly, I think it is a hardware issue.

After upgrade the firmware, you should hard reset it, and then reconfigure it. Sys light should be flashing on. When it happens, please check status of your indicators.

As I know, TL-WR641G is a discontinued product. Because your version is v3, it cannot be upgraded with v4. It may cause damage to your device.