In what country are you?
Could you send me the ip address of your DNS server? (you find it in System Preferences > Network > Advances > DNS.
Can you try 1.1.1.1 or 8.8.8.8 as you DNS server?
Do these two links work?
https://updates.glyphsapp.com/latest2.php
https://updates.glyphsapp.com/Glyphs2.6.3-1271.zip