Firmware verison of your WA750RE is up to date? Universal mode should be the right one. Besides, according to your description, the RE is running as well. That WA750RE is using smart IP or static IP...

Which firmware did you upgrade? The one for TD-W8970 V3? Sounds like faulty. If it is out of warranty, you have to buy a new one. :(

What's the brand and model number of your main router? No wireless MAC filtering rules? Range extender mode 1 is selected, right? How about status of lights on RE after you finish configuration?

Model : TL-SG1008P? Did you try different POE ports on the switch to connect those two cameras? You should contact camera's technical support and check if they are under warranty.

It was working, right? Did you get a WAN IP address on web interface of WR841N? Did you reset the router to factory defaults and reconfigure it?

Provide the topology. What's the firmware version of your R600VPN?