Last month it was revealed that Google Cast-supported devices were causing networking issues for many. After the story garnered widespread attention, Google quickly pushed out a fix in the form of the Play services 11.9.74 beta. Affected parties were able to manually install this version or jump into the testing program to get it, and it was supposed to fix the issue. But, that was a beta release that not everyone received (since betas are opt-in). The beta seems to have gone well, though, as Google has now pushed these fixes to everyone else via the latest stable release, v11.9.75.
This update may also contain the fixes for the Nexus Player remote sleep issue, which were claimed to be server-side but may also require the updated Google Play services to work. It's also a bit unusual to see a beta hit stable so quickly. Typically the beta versions run a few numbers ahead of the stable builds, but Google may have fast-tracked this release for the Wi-Fi fix.
Since it's now in stable, you should be able to pull down the update over the Play Store, though you can also download it over at APK Mirror (for Android, Android TV, and Android Wear), if need be. Hopefully, this is the last anyone has to hear of the Wi-Fi problems associated with Cast-supported devices.