iPad cannot connect to cellular network

I am using 11” iPad Pro 2018. It cannot connect to a cellular network. I have tried to reinsert the SIM card and restarted several times. Also reset all network settings. But still does not work. No SIM card can be used. I contacted the network operator where I bought the iPad from, they confirmed the iPad is free from any network or bandwidth blocking. I am also using the latest iPadOS 13.7.

The only time where the cellular icon appears on the corner of the screen is after turning the iPad to Airplane mode and turn it back on. The icon displays for less than 10 seconds then disappears.

In the menu Setting, General, About, displays Carrier iPad 41.0 with no cellular data number. However there is a valid SIM card inserted. The SIM card can be used with no problem when installed in my iPhone 11.

Please help. Thanks.