If you do not know the password of a device, the standard way to correct is to reset it to factory settings and re-enter all of your settings. See your manual on how to reset to factory. Most times...

That is strange. Be sure WIFI is off on the MacBook and the MINI. Will it work correctly if you power down the router connect only the mini and the Airport then power it up without the MacBook?

The TL-WR340GD is just a router. You will need a modem or some type of device to connect to your internet service provider (ISP). Your ISP will have a list of devices (modems) that will work. Others...

DDNS is a service that is used mostly for port forwarding so you can see the device on your network and from a WIFI network other than you own. For example, a Camera you may want to see both on your...

Things to try... Reset to factory and re-enter your settings manually (not restore from backup). Swap the C5400s so the one that is not working is in place where the working one was. Ensure both are...

Back to my orginal advice. Switch the router out with a new one.