According to the wiki:
I have
WebUI installation fails with FW upgrade/backup fail, check FW file... 
I tried OpenWrt versions
How should I proceed? Try another method? Open up my device? Start soldering?
Thanks for any advice!
If I remember right newer firmware became signed so they can’t be manipulated.
Thanks @flygarn12 . Where can I find more info on this? Can the signature check be bypassed/disabled?
Sometimes you can try to downgrade the vendor firmware.
Other option might be using serial and tftp to boot into initramfs and flash Openwrt from there but might need some experience, confidence and the fearlessness to be able to brick the device....
Most info is probably here in the forum in different treads.
As far as I have understood with this D-link device family is that it pretty much became a dead end in the Realtek project when the signed oem firmware showed up.
And the Realtek switch dev tread is pretty quiet nowadays.
So the only option is to get an old one from eBay then? That's too sad. I have 2 devices from like 2 years ago and should consider myself lucky then? 
You can try the tftp install method. But doesn’t that work you probably need a F1.
But more precisely, you actually need a device with old firmware that isn’t signed.
But the problem with D-Link is that it is a continuous moving target where you never know what you get until you read the sticker on the back.
I already have mine running with Openwrt...
My comment was more ment in a sense that I have to stick with mine and not go out and buy another one from the shelf because chances are that it's now delivered with the new signed firmware version...
The main problem is that no one has played with >v6.30.xx so far. Everyone who has an older device with <=v6.20.xx installed shouldn't dare to install the newer OEM version, no one who got a newer device has done serious investigations about the new situation so far. The OEM changelog doesn't sound too promising, talking about 'encrypted' (probably signed) images - and it being impossible to downgrade.
Serial console access will be needed, maybe you can still get bootloader (or at least shell-) access that way. Reflashing the spi-nor system flash externally (with the old firmware) should be possible (but non-trivial), as the hardware itself presumably hasn't changed (significantly). Please document your findings, missing information is the biggest hurdle for these devices (and upgraded older ones) so far.
Only one thing is clear, the situation won't fix itself, unless someone who is 'blessed' with an affected device starts digging into it. It's not going to be impossible, but it may be rather convoluted.