IPad will not connect with extended network and/or guest network

Question

I have extended my main network (AP Extreme) with an AP Express and uses a guest network too, all Macs, PC's, iPhones, game consoles, mediaplayers can connect to the internet with both networks. Only one device does not work with the main network and that is the iPad (1st gen). It can connect to the guest network, though. If i disconnect the Express, the iPad can connect with the main network or if I set the guest network to off it will too. But  I want to connect to the main network (remote access).
Device.                          Network.          iPad internetconnection
Extreme.                        Main.               Yes
Extreme + Express.       Main.               Yes
Extreme.                        Main + guest    Yes both
Extreme + Express.       Main + guest.   NO
I have reset the network configurations on the iPad and it works for only one time, if the iPad goes into sleep mode and restarts, the connections is lost. The iPad seems to be connected with the router/AP but no internet access.
Fyi
- all devices have a fixed IP address dedicated by the Airport. i tried to use DHCP, but no difference
- radio channels on automatic
- all on 2,4 GHz band
IS THIS A IPAD 1 ISSUE and do I have to accept the fact that Apple does not support the iPad 1 anymore?
Any suggestions how to fix this? Thank you.

Answer 1

Thank you for the suggestion.
The name of my home wifi network is automatically found by the iPad and after I select it the iPad asks for a password.  I have typed in my password that I use with my laptop, but it does not work.
I also tried the two alpha numeric 13 digit strings on my router.  It is actually a Belkin.  Neither of those worked either.
finally, I went into general as I could not find settings to tell the iPad to forget network settings?