Maybe it is a magic router that knows it is outdated. Is that the actual part number you are talking a router that is close to 20 years old.
Not sure those older routers had a lot of strange issues. If you can get newer firmware I would try that.
So you never want to use WEP even if you can get it set. WPA is somewhat better it is still not very secure but I would not considered it cracked like WEP is. Some end devices does not support WPA or you must override them to get them to support it since it is not a recommended encryption.
The largest problem is that is going to be extremely slow using 802.11g