What's the model number for your router? If you don't use the computer on the second floor, will you still have the problem? You can do a test, maybe it's the PC on the 2 floor take too much...

Don't use the smart connect function, it's not so smart, not better than you choose a connection by yourself.

only the speedtest is not working ? How about the internet, if you connect to the wifi or hardwared to C5400, can you browser? I recommended you to go to a speedtest website and test the speed,...

Who is your internet service provider? How about IPV4? Does it work fine?

Yes, the name should be renamed as wr(wdr)xxx(nd)_tp_recovery.bin like this format, for your model, it should be Archer VR1200vv1_tp_recovery.bin

It seems it's the switch which influence the AP to work. Is your switch a plug and play switch or L2/L3 switch which need to to some settings? On the switch , there should be serveral different port,...