@Tonya 

In Settings for the WIFI, open up the connected one and you'll see PRIVATE ADDRESS. Turn that OFF.

With it on, it will provide a different MAC Address to each SSID you join. It will also CHANGE that MAC Address every 24 hours (Apple states that it will use the SAME one for the SAME SSID though, but I've seen some complaints that it doesn't) and that might cause the need to re-enter passwords. This setting option is NEW for iOS 14 which you might have upgraded to recently.

Device can change SSID's automatically though depending on signal strength. Doing so could cause the need for the password it seems.

Tonya wrote

I am using the guest networks (I don't know how to connect to the others)

 

Devices will normally connect to the same one they did before.

However, on the iPhone in Wifi Settings you should see ALL availalbe networks. All you need do is select the one you wish that isn't connected to now. The iPhone will then try and connect and ask for the SSID password. Supply that and you should be on that SSID. I would have the GUEST SSID(s) on unless you needed them. Depending on Guest SSID setting, you might now have full LAN access, or if the setting is to allow it, anyone getting on that SSID(s) would have access as well. Guest SSID's were meant for guests in the home you don't want to see all the LAN devices, only have Internet access.