Steam client, dd-wrt, privoxy: no internet connection

[Dingo]

I recently had a maddening week trying to overcome an issue where the Steam client (suddenly, one day) started reporting no internet connection on multiple PCs in my household. I'm describing the fix I discovered in case it may help someone else in my situation--and also because I can't find a way to send this to Steam support for inclusion in their troubleshooting docs.

The symptom I suddenly began seeing was a refusal for the Steam PC app to start up or sign in, saying Steam needs to be online to check for updates. On the other PC, it would sign in after multiple attempts. But on that machine, trying to download a game would fail with a "no internet connection" message in the download manager.

I tried everything in the Steam docs, and much more besides. Clearing the cache, moving the download directory, reinstalling Steam (multiple times), adding "-tcp" to the startup parameters, using steam://flushconfig all failed.

Strangely, both PCs could run Steam fine in Safe Mode. But since games won't run well in Safe Mode, that wasn't an option.

Just as I was thinking about reinstalling Windows or building a new PC, I tried a spare router I was lucky enough to have sitting around. And it worked.

Then I found this:
https://www.reddit.com/r/DDWRT/comments/mhjae9/disabling_privoxy_starts_to_break_steam/

The solution was to enable the ad-blocking service on my dd-wrt router but configure Windows not to use the ad-blocking proxy. It doesn't make any sense, especially given that Safe Mode gets around the issue, but there it is. Hope it helps someone some day.

tl;dr: If you use dd-wrt, enable Ad-Blocking but configure Windows not to use the proxy.