Please read my comments in total:
→ If you download the packages and run the dnf command from the very folder, it will find the packages contained in the folder.
Concerning the links …
https://codecs.fedoraproject.org/openh264/39/x86_64/os/Packages/o/openh264-2.3.1-2.fc39.x86_64.rpm
Here are all packages of this repo: Index of /openh264/39/x86_64/os/Packages
When you change 39 to 40 in the link, you can see that 40 does not yet exist.