My first thought would be the update traded the last bit of speed for improved stability - for this is what the 9020s have been lacking.

PLCs do not have IP addresses to begin with. They can only be found by PLC utility programs that use the specific protocol to talk to PLC devices.

TL-PA8030P, with firmware from March 2016 at least, handles multicast correctly on the powerline and integrated LAN switch. Multicast on WLAN is much more difficult, and a recent test in Germany...

I would - but mind, the V1 with their current firmware, even though old, work very well at least for me.

Pair them again, this time not using the default network name. If one of your close neighbours did the same thing, you may end up all on the same PLC network - and that'll seriously disturb DHCP and...

Try whether WPS works for other client devices, e.g. a smartphone. If that doesn't connect through WPS either, then you need to do this differently on the router side.