WDM4.9.1 doesn't detect D90D7 Thin Client

I disabled the firewall on the client and the server and was able to ping between the two, but WDM would not detect the client.

Have you tried to disable the firewalls first on the D90D7? also do you have firewalls running on the WDM Server as well?

Dell will deny it, but WDM is horribly broken, even in 5.0.  I encourage you and others to keep after them in these forums and elsewhere, until they start to seriously address the issue.

• Discovery simply does not work as it should, whether subnet, MAC, or otherwise.  As noted above, this is likely some combination of Dell continuing to ship brand new clients with years-old agent software, later versions of WDM not being backward-compatible with previous agents, and the complexity they've built into the whole multi-client-agent design.
• You'll probably need to go thru some combination of: updating the baseline firmware on the client (probably by a physical USB stick!); multiple reboots and attempts to get it to discover; pushing an update of the boot agent; pushing an update of the HAgent; pushing an update of the WCM agent; MANUALLY setting the WDM control panel settings on the client to point to the specific IP address of the WDM server, thereby basically bypassing any sort of intelligent "discovery".
• Once you've got it working, you'll basically need to repeat all these manual steps and workarounds for N-number of thin clients that you wish to deploy, or if you in any way deviate from the very narrow functional "happy path" that the software assumes you know how to follow, with little or no documentation.

Good luck.