Where can you access WattMan with RX5700? Also, wouldn't using it cancel the warranty on the card? Maybe it would just be safer to return it? I mean I shouldn't have any problems running 3 years old game on this PC...
Underclock does not affect the warranty. I have not given you a suggestion that will damage the warranty.
You can go to WattMan with the game settings section on Radeon Software. And you can try to lower the GPU frequency a bit. -5% or -10%...
PSU: SilentiumPC Vero L3 600W (SPC266)
This is your PSU. AMD graphics cards can cause problems with some PSUs. This is not due to the PSU, but problems may occur. There is a possibility that you may experience this problem for other GPU manufacturer as well. For these reasons, choosing a powerful PSU and a GPU with good VRM will reduce the chance of problems. So it is necessary to be careful when purchasing hardware.
Sorry if this is reviving a post or if OP has already resolved their issue but I found something else that may be helpful. I had issues with CoD MW(2019) where computer black screens and computer becomes unresponsive so I have to force shutdown by holding power button and starting PC again. I looked up the Asus Dual Rx 5700 EVO support page and saw there was a Bios update version 01/22/2020 that provides "improves stability." I bought my Asus Dual Rx 5700 EVO right when it came out so it had a previous Bios version. I updated it with the manufacturers Bios update winflash tool and now no more black screens so far.
Disclaimer: flashing bios is potentially risky and could brick your gpu. Do so at your own risk.