Problems with display - trying to upgrade from Win7 to Win 10 on my Z420 Workstation

[Mancunian_Nick]

Hello everyone. I do hope someone can kindly help me with a problem I have.

I have a Z420 workstation PC and now have 2 monitors. I moved my old AOC 21" monitor, connected as before with a DVI-D to DVI-D cable and installed a new LG 29wk600-W ultra-wide monitor using a DP to DP cable and as far as that goes, all is tickety-boo and working just fine.

Using Win7 Display - I have them currently set up as extended display using the new LG monitor as my main monitor (1) and the old/AOC (2).

So for hours I've been trying to use either a RAM stick and/or DVD drive installation disk for Win10 without success. I've been on the phone twice to LG who were unable to help me at all - couldn't speak to a tech guy so had a lady from customer services playing piggy in the middle and trying to be a repeater between the 2 of us which is crazy but he wasn't 'customer facing' so that wasn't possible but anyway that's irrelevant really so ...

After I spoke to LG for the last time, just out of curiousity, I switched on the old monitor and knock me over with a feather but all the start-up stuff - bios and so on, were there! Of course the new monitor had turned off - "no signal - entering the power mode" - and because it was my last attempt to upgrade using the old monitor that caused me day's worth of problems and unable to use the computer, I was damned if I was going to attempt it again, although this time I've taken every precaution first - created a repair disk, 2 installations on DVD and RAM stick, an image of the C disk on my D drive and so on.

SO my question is this please - and thank you for bearing with me to this point - why are those still going to the old monitor despite my setting the new monitor as 1/main monitor rather than to the new one? And how can I make it so all that goes to the new monitor instead? I won't have any display issues now and surely there has to be a reason why currently it's going to the old monitor rather than the new? It can't be the connections and/or cables surely? They're all new! I just don't understand what it is so unable to correct it. Sounds simple enough on paper but I'm not very au-fait technically with computers other than a basic understanding but I am ok with software and so on so any help or advice anyone could kindly offer would be *VERY* much appreciated.

Thank you kindly, Nick.