Be careful here! OS9 has very little protection from internet malware & viruses. I don’t think there is any 3rd party antivirus/malware apps that even has newer definition files. If you are connecting to a local server that is about all I would do with it. OS9 also doesn’t offer stronger WiFi signal encryption (man in the middle attack). Maybe it’s best not connecting it at all and use it as a writing station using sneaker net (USB thumb drive) or an Ethernet connection to get to a printer.
But as you asked here’s likely your problem.
First your system uses an older 802.11 standard 802.11b, so your routers AP port needs to be able to support it and is configured to accept connections from it (some have it disabled by default as its less secure!)
You can also encounter issues when you don’t have your WiFi bands named uniquely. As an example my routers name is ACME I would rename the AP’s as ACME_2.4 for the 2.4 GHz band and ACME_5.0 for the 5.0 GHz band. So I know which band the system is trying to connect to (your older AirPort board only offers 2.4 GHz)
Then we get into the encryption & passwords, WEP Wired Equivalent Privacy is the only option if I remember correctly so your router need to have it setup and password is often limited to lowercase & numbers and no spaces or other symbols.