That router (CT-5372) is bridged. You are being assigned the external IP address (172.29.97.XXX) so it has to be bridged.
You can not ping a router that is bridged. This also means that your server settings that we tried earlier should have worked because any port forwarding/DMZ would have not interfered with the network bridge.
EDIT: If you want to test to see if DIR-615 is not working properly with the server here's what I'd recommend doing.
1) Unplug everything from CT-5372 except the line-in from the ONT
2) Plug your server computer directly into CT-5372
3) Run ipconfig /release and ipconfig /renew
4) Open a browser and make sure your internet is working
5) Disable all firewalls on the server and then start srcds
6) Use canyouseeme.org on your server port and see if it works
If it fails then your ISP is blocking that port.
If it succeeds your DIR-615 is crap.
you have to be confusing him. you can indeed ping a router that is bridged. if the DIR-615 is in bridge mode it means that the CT-5372 assigns an internal IP address to all devices, including the DIR-615. (i just pinged both of my routers which are set to bridge mode.) all devices are then within the domain of CT-5372 and all devices can see one another. when in bridge mode, you cannot port forward or set any firewall options as it would conflict with the entire LAN. this tells me that the DIR-615 is setup as a regular router and is forwarding ports only to be blocked by the CT-5372!
if you are plugged directly in CT-5372 and the problem persists than it is not the DIR-615. it seems to me that the CT-5372 is assigning your machine your external IP address which means that the DMZ is on OR it is some strange setting your ISP has when it installs those routers.
if you cannot login to CT-5372 using 192.168.1.1 like the manual says AND you are plugged directly into it then you should call your ISP and ask...
#1 are you blocking any ports using the CT-5372
#2 what is the default IP address to the CT-5372
call them now. they have 24/7 support. go go go !