Most phones have a mobile data auto switch. When there is no Internet or a poor Internet connection it will switch over to mobile data. This poses a problem connecting to the SSID or mobile UI IP address as it will not properly connect. You will need to disable the auto network switch/mobile data switch. Below are some suggestions for some software builds, but note they will vary from build to build. Please consult your manual or manufacturer to determine how to turn the auto network switch off.
Go to Settings > Cellular or Settings > Mobile Data. Then scroll down and tap the slider for Wi-Fi Assist