If you obtained your router from an ISP, they are the only official source.
Sometimes, ISPs remove older firmware links, but they remain cached. Use web.archive.org to search for strings like zte f670l v90 fimware.
Sometimes the zte f670l v90 firmware link simply does not exist for your specific ISP-locked variant. In that case, you have better alternatives:
This is the easiest and safest way to update.