@Evilcraig Replying to my own comment - I removed the v1 from the network, created a new network for the v1 only. Once it was in it's own network, I reached tplinkdeco.net and could upgrade to 1.5.1...

I have 3 x M9v2 and 1 x M9v1 where the Main Deco is v2. They all had 1.5.0 installed. I wasn't sure how to go up to 1.5.1 with 2 different firmware files available - I was nervous that the v2...