Which router do you have? You didn't say in your post.
What device are you trying to connect to it? (Make, model)
802.11b is absolutely ancient. Like late 1990s ancient. It is also very slow and not secure. Some routers do still have support for it but not all. It is best to avoid 802.11b at all costs.
802.11g is still common but it depends on the encryption the router is using and what the device you are trying to connect supports.