Maybe put out a request for help from someone nearby that can come help maybe, order some pizza for em as a thank you.
Is the psu fan spinning, if so then its switched on so should be working hopefully.
Is the cpu in properly, any damage on cpu?
Is the memory in ok
Is the cpu got a onboard gpu if so run the cable from mb to monitor for testing. So no gpu for now.
Is the mb a diff make or brand or same exact type?
Got any old parts that could be used to test with, old psu, old cpu etc?
Failing that, take it to a nearby computer shop that has a good rep for repairs etc (if u can) and say theres 20 quid can u make it turn on.
Or as i said before, see if u can put a request topic saying u need a hand and ud like them to bring some parts like psu gpu that they have spare that they can put in to test to rule out stuff and pin point what is the issue part.
My guess would be psu or cpu since ur having issues with the new mb like old one.
Last last resort, do a few video shorts with ur mobile on utube and show what u tried visually and explain that way, might be more helpful than txt chat. I dunno.