Have you set your iPhone and iPad to NOT automatically join networks?
Are you connected to your home router 2.4g or 5g?
Have you tried disconnecting from your home WiFi and using the WIU as the connection point? The iPhone and iPad will connect to the WIU WiFi
FYI. I am able to use my iPhone 12 Pro and iPad 7th Gen at the same time, using the WIU AdHoc WiFi network
I put them both in Airplane Mode so nothing tried to interfere (calls, etc..) I turn off automatic updates for all apps, etc..