First the why.
I established the original connection to the ISP with just the DSL-300G+ modem connected to the NIC in the PC (i.e. NO router in the set-up). The ISP uses the MAC address of the computer (i.e. of the NIC) to authenticate the connection. When the router is in use that original (PC NIC) MAC address needs to be "cloned" through the router (i.e. as far as the ISP is concerned the original MAC of the original NIC has called in again and the connection is authenticated.) (This is further complicated when now the original Ethernet LAN card NIC is replaced by a wireless card NIC. Still the ISP expects to see the very original MAC address coming in the connection requests; without it the connection fails authentication. My ADSL ISP uses this and many cable companies (e.g NTL) do too). So the Netgear router (in common with many routers) has the facility to "clone" a MAC address.
So to the how.
The router has a facility to use enter MAC address other than the default which is usually the routers own address. If you have a copy of the WGRT614 user manual (wgr614_ref_manual.pdf) then goto pages 2-12, 2-13 & 7-7. Especially Figure 2-11 on pg 2-12 does the business. At the bottom of the screen dump under "Router MAC address " you are offered three options. Use either the 2nd or 3rd options (the former if you are still using your original NIC in your PC, the latter if that NIC has been replaced by another. In my case I take the 3rd option (entitled "Use this MAC address") and I manually enter the required MAC address. Use the Apply button to commit the change (and it may be necessary to restart the modem followed by the router).
I'm not saying this will cure your problem but there is a fair chance.
One other point: the DLS-300G+ and the Netgear both come with a default web-access GUI IP address of 192.168.0.1 Note that they cannot both operate, obviously, on the same IP address. I opted to leave the DSL-300G+ on 192.168.0.1 and adjust the Netgear to use 10.0.0.1 (and thus it issues DHCP addresses to the 10.0.0.0/24 sub-net. This need to have different sub-nets gets a very obscure mention in D-link documentation (dsl-300g+_qig.pdf, page 8) which describes how to get the DSL-300G+ modem working with the Dlink DI-614+ router.