After being tired and frustrated as I also lost https access to my internal resources from WAN (in other words FW rules for inbound connections also started to misbehave) I pulled this router off my services and put back the old DIR-655. All is working back as it should.
What I gained by doing this:
1. NAT looopback issue is gone.
2. All inbound access via port forwarding to synology devices established back.
3. DHCP service is working very good.
4. Have a DMZ option if I needed on DIR-655.
What I lost by removing DSR-250N
1. lost 4 physical RJ-45 ports ( I miss this so bad)
You an always add an external network Gb switch to add more ports. I use a 24port on with my router.2. Lost ability to do VPN at the router level
3. Lost ability to create VLANs
4. Paid for something I cannot use and since I'm over 30 days on my purchase the store would not take it back.
Not sure why a business router cannot handle home networking. One would expect the opposite to be true.
Oh well. My only hope is if later this can be flushed with dd-wrt and behave as it should.
This is a business class router. WRT will never be available for it. Only a few DIR series routers have WRT made available to them. Maybe someone can review your router settings with you using Link>
teamviewer if your interested. Its safe and secure.
Did you phone contact DLink support to see if they could help you out?