They are different networks . The answer given by DLink support probably refereed only that it should work because they thought that the client from which you're trying to access the USB HDD is connected to the DLink router ( with an IP of 192.168.100.x ) and not the other router ( the modem is a router also ) and in that scenario indeed it should work .
Sorry I didn't understood what you mean?
Actually I'm accessing the internet through Dlink router that is connected to a Modem. I'm trying to access a USB that is connected in the D-LINK router. This is no secret... Any noob can do this... I did this all my live with the Linksys WRT54G that is with the DD-WRT that I installed and configured.
I used a TP-link router that worked perfectly with the USB sharing for 2 weeks. The issue was with the TP-LINK support and the IFTTT that didn't work it. I am very picky with the products I buy.
You could try to rewrite the original firmware in recovery mode to make sure there isn't an issue with the firmware you have now (if there isn't a real hardware problem it should fix the issue preventing you to access the USB device root ) ; DD-WRT by default is intended for more experienced users and at this time it's not at the level of stability of the official firmware ( especially on the 5 GHz wireless part ) .
I will try it now...
Honestly, I won't waste my time any longer if this doesn't work.
These routers are designed to work with novice users. And there should be no secret in all this.
Thank you!