It's been referenced in other posts a long while back, but I'm putting it here, also. Insider build 14342 is the first build where I have had an issue adding the Optional Feature .net Framework 3.5. It fails with one of the following error codes:
0x800F0906, 0x800F081F, 0x800F0907

It tried to download the files for the feature, but was unable to locate them on any MS server.

There is an excellent MS article explaining and giving resolutions.

I used the mounted ISO created from the install.esd of 14342 and ran the following command at admin cmd:

DISM /Online /Enable-Feature /FeatureName:NetFx3 /All /LimitAccess /Source:d:\sources\sxs

Where the Source:d was the mounted ISO drive letter I used to do the tech review installation. You could use a DVD, also.

Hope this helps anyone who has encountered this issue. TC