So, I found 20 minutes when nobody was using the WiFi and disconnected everything.
Reset the router, then update to 1.03WW, then another reset, all this with the HDD disconnected. When I configured PPPoE, I saw in the figure the place for the USB, which wasn't before.
I connected to the internet (PPPoE), reserved an IP for my computer so i can have port forwarding, connected the HDD, enabled UPnP, and then connected the TV to the WiFi network (5GHz, as it is on the other side of the wall with the router), and voila, I can access the HDD!
The internet connection works well, ~94 Mbps download / 95 Mbps upload, and my connection is 100Mbps, so it's 95% fine.
I'll monitor the WiFi connection for the next couple of days, to see if some devices disconnect again repeatedly (hope not). I'll update my case with D-Link support, to be fair with them.
Until now, only after upgrading the FW I have been resetting the router, I woudn't have known that a reset BEFORE upgrading was that important.
Thank you very much for your help, I hope I won't return here with the same problem
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I'll be monitoring this forum, though.