They just sell them in a KIT, so I bought another KIT, one connects to router, the other three are in other rooms, they work fine, so you can also buy another KIT

the AV500 is not NAT product, so it shouldn't affect that, you may test the WiFi connection of the extender as well

the SR102 is just a router or a modem router combined? if it is just a router, you may replace it with a computer and check whether computer can get internet without the router, if it is a modem...

Is the powerline led light up now? if not, you may try to pair them again via the pair button, if the utility doesn't detect the powerline devices, you may check whether the Ethernet led light up...

As far as I know, all the TP-LINK powerline extenders are using HomePlug AV or HomePlug AV 2, the AV2 is backward compatible with AV, so any TP-LINK powerline extender would be fine such as...

the speed on the utility is powerline rate which is different from internet speed, and the TL-WPA7510KIT supports 1000Mbps for powerline rate, so I think the problem is caused by some powerline...