This seems to work for me: https://www.reddit.com/r/MacOSBeta/comments/1re3stp/comment/o7tuq0u/

See also https://www.reddit.com/r/MacOSBeta/comments/1rhado9/macos_tahoe_264_beta_2_partial_internet_some/

Strangely it's just the web browsers, not other apps like for example WhatsApp. Deleting and reinstalling Firefox & Chrome didn't help.

Same problem. Since the update I can't get Firefox or Chrome to work, neither with WiFi nor via personal hotspot of my phone. Safari is working, but not fast. I left the Beta program because of this, but apparently there is no way to switch to the not-beta variant until there is a new version, which might be in a month?