It doubled all the MAC addresses causing a pain to do MAC filtering. It also resteicts internet acces for the trafic going through the ethernet port, but i could live with that. I don't really need my sensors connecting to the outside of my home. Still it would be nice if I could use it with my phone.
Quite some time after the above failure to expand my network, I came across the Linksys RE6300.
It was double the price, but that also gave me some hope it would perform accordingly better. And it did! It actually works exactly as advertised. It creates a second network that is completely transparent and acts exactly the same as the main one. Even better, I added a second extender, connecting it to the first ones wi-fi signal, and it also works great.
Now I have all three floors of my hose evenly covered, plus I added a wired acces point in my garden shed, to cover the outside of the house. Happy times
I have this setup now for a few months and it is reliable, stable and plenty fast for my needs.