You may want to try disabling DNS over https as that was causing similar issues for me https://support.mozilla.org/en-US/kb/firefox-dns-over-https
+1 for Syncthing pretty easy to set up and get full control over your synced files.
You may want to try disabling DNS over https as that was causing similar issues for me https://support.mozilla.org/en-US/kb/firefox-dns-over-https