[DGL-4300] Download/File transfer freeze

I recently bought a new D-Link DGL-4300 router, it is really a great product but has a big problem transferring files with wireless!
When I try to download a file, it starts but after a while it suddently hangs.
It happens the same when I try to transfer files from or to other computers of the LAN.
If I open the logs of the router there are a lot of entries like these:

"Blocked outgoing TCP packet from 192.168.0.5:1828 to 130.192.56.9:80 as FIN:ACK received but there is no active connection"
"Blocked outgoing TCP packet from 192.168.0.5:2434 to 159.153.235.26:80 as RST:ACK received but there is no active connection"
"Blocked incoming TCP packet from 159.153.235.76:80 to 1.81.106.153:2398 as ACK received but there is no active connection"

I don't have SPI active on the router or a software firewall running.
It seems like the router deletes the NAT entries before the transfer is finished, and it obviously can't find an active connection related to the transfer.
What can I do?
I asked for a new firmware and for support but D-link doesn't answers....

....nobody knows?

I really need help!! nobody can suggest me something to try?