If you are trying to register or log in to NTSA TIMS portal and your serial number does not match your ID number then it means that at some point you might have replaced your ID. When you replace your ID, your serial number changes.
If you start contacting NTSA online, it might take time before you get help. The best thing to do is to visit NTSA office in Upper Hill or near your area to have your ID details updated.
Here is the bus direction to NTSA Office in Nairobi.