its only running on an SSD - nvme.2 to be specific

I can't powerup at all

I read in another thread, that I should take out the battery and hold down power button for 30 sec, reinstall memory and harddisk and plug in AC and see if it boots up.

Should I do that?

(It was actually you who posted that answer I see now )

Is the operating system running from a solid state drive, or a spinning hard drive?

And if it's on an SSD, is there also a spinning hard drive in the system?

If you can get to the Dell diagnostics (F12 at powerup), run a full system test.

So it seems the motherboard is bad. I did everything you described, and its only the fans that starts up.

It doesn't make sense to switch out the motherboard, or would that essentially be the same as a completely new laptop?

Try removing the battery to see if it will run on just the adapter. That model is from 2017 and if you have the original battery it's lasted longer than normal. Recent models can run on just the adapter so it's worth a try. AFAIK-There are no replacement batteries for your model available from Dell or Parts People. Avoid buying replacement batteries from private online sellers on marketplaces. Most are scams. More info- FAQ - Laptop battery articles - Dell Community

If using the laptop as a desktop replacement suits your needs, you can use use it that way until buying a new computer.

Unplug the system and disconnect the battery as you describe.  Hold the power button for 30 sec.  Then remove and reinstall the memory modules and the SSD.  Leave the battery out and see if it'll power up on AC only -- if it will, but not with the battery installed, you've found the problem.

If it still won't power up, remove the SSD and see if the system will POST.  If it will, you have a faulty SSD.

If not, the mainboard is bad.

A board will run $350 and up -- depending on the CPU and GPU aboard, it could be twice that or even more.  On a five year old system, I call that a questionable economic proposition, but it is still a fraction of what an equivalent new system will cost.  Only you can decide the economics.  If you have to pay for labor on top of the part, it tilts the calculus toward replace rather than repair.