WA850RE and WA7210 are both running in latest firmware?
WA7210 is in access point mode connecting to LAN port of W8961ND?

4. The AP & RE connected together and RE broadcasting the signal, but when I try to connect to the internet, there's no connection.
If so, what's the IP address obtained?

You topology:
----internet-----TDW8961ND ))) ((( TL-WA7210N ))) ((( WA850RE ))) (( client devices such as laptops

In the above topology the wireless signal is repeated twice before reaching your client devices, which is a bad idea because the wireless performance will be sharply weakened when being repeated twice, as well as causing many unexpected MAC layer transparency issues such as DHCP/ARP problems.

Suggestion 1:
Upgrade both 850Re and WA7210N to the latest firmware from www.tp-link.com/en
On TL-WA7210N, navigate to wireless menu --> enable WDS option.
On TL-WA850RE, navigate to wireless menu --> select mode 2 (WDS mode). Do not use mode 1(universal mode)

(I believe this is the solution. If not, check the below suggestion)

Suggestion 2:
TL-WA7210N is a powerful directional access point which focus its signal towards one direction. You can just connect 7210 to your modem router via Ethernet cable and use the 7210 as an access point to cover a long range. Then you can use TL-WA850RE to repeat the signal from TL-WA7210N if necessary.