when restoring from sleep, no video signal  

Pleased you fixed it
The problem with installing in MBR mode is
1. Early Launch Anti Malware protection does not work in MBR mode
That protects the computer to some extent from malware executed at boot
2. Firmware setup and changes are accessible from settings - advanced options - restart
3. Start is usually faster
4. larger disks are allowed
5. Generally windows 10 is smoother performance overall
See on link
https://docs.microsoft.com/en-us/win...ences/oem-uefi
UEFI benefits
Firmware that meets the UEFI 2.3.1 specifications provides the following benefits:

Ability to support Windows 10 security features like Secure Boot, Microsoft Defender Credential Guard, and Microsoft Defender Exploit Guard. All require UEFI firmware.
Faster boot and resume times.
Ability to more easily support large hard drives (more than 2 terabytes) and drives with more than four partitions.
Support for multicast deployment, which allows PC manufacturers to broadcast a PC image that can be received by multiple PCs without overwhelming the network or image server.
Support for UEFI firmware drivers, applications, and option ROMs.

Cheers
It cannot possibly void the warranty and indeed it should never have been installed in MBR mode
You will most likely have to do a clean install
However there is a way to keep everything but it is NOT always successful
Convert Windows 10 from Legacy BIOS to UEFI without Data Loss

The safe way is to make a Macrium Image first on an external drive, so that if it fails, you can restore the image and you are back as you are now

If you need guidance on that please just ask but the full guide is here
http://updates.macrium.com/reflect/v...df?src=sidebar

and if you do not already have it installed the free edition download which will do the job well is here
Macrium Software | Macrium Reflect Free

click the home use button
you do not have to enter an email just click continue

Make a full image of your drives and create the rescue media on usb.
Then when you have that follow the guide to convert MBR to UEFI

If it works you are good to go if it fails restore the image

I had a similar issue which was solved on advice from Viewsonic by editing the registry and setting EnableUlps to zero under HKEY_LOCAL_MACHINE\SYSTEM\Setup\Upgrade\PnP\CurrentControlSet\Services\amdkmdag

How can I be sure it will install in UEFI mode? If I enable UEFI in the motherboard first, will it just automatically do it?

Not a problem you go here
https://www.microsoft.com/en-gb/soft...load/windows10
you download the tool following then the procedure to create the USB
on the section download tool now
so that is not the blue bar - update now but the next one down on the page
you then expand the section - download the tool now to create installation media
and follow this
Follow these steps to create installation media (USB flash drive or DVD) you can use to install a new copy of Windows 10, perform a clean installation, or reinstall Windows 10.
use a usb flash pen - not a DVD

Then when that is made you go to this link and follow the procedure for a clean install on uefi
Clean Install Windows 10
Please read it carefully

On the main points note point 3
Boot to the UEFI firmware settings for your motherboard, and enable or disable UEFI or Legacy BIOS (CSM) mode for how you want to install Windows 10.

So you are setting firmware UEFI
then point 5 boot from the tool

You should be able to that on the one time boot menu without going back to firmware setup
If there are two boot options for the usb pen then one will show the pen as UEFI and one will show it without the UEFI
You of course want the UEFI one

This is shown at item 4 on option 2 on this link
Boot from USB Drive on Windows 10 PC
which is in fact at point 5 on the installation guide

Then when you have it booted from the usb you return to the guide and follow
the procedure from point 6 to point 13 and then YOU go to point 15
How to Select Drive to Install Windows 10 with UEFI
and most importantly you delete all partitions on the drive -as shown on the image on the link at point 15

you will then end up with the partitions shown in the image further down the page within the yellow background area of the guide.
NOTE that your recovery partition will not be shown at that stage but created late in the install procedure.

ANY questions please ask, the install procedure for the edition you are installing the latest available needs to be followed very carefully unless you want to accept microsoft default options for example

For instance see item 25 on the link. Local account and Microsoft account. If you want local account you have to install without internet connection and you will see many other options such as back up with One Drive etc
The install really does require you to take your time and make sure you are not simply clicking on NEXT at various stages.

mike
NOTE although you say there nothing you need to save - do not forget to save your personal data
pics, videos, docs, contacts, etc from your user folder

You do not want Compatibility support mode
You want UEFI mode

https://www.biostar.com.tw/upload/Ma...IOS_180803.pdf
page 20 CSM disabled