Internet: automatic access point disabled Why?

I use the access point of my Android phone to work surfing the Internet on my HP laptop …….

I am at my desk and after about 20 minutes, my phone automatically turns off the mobile data and I get up from my desk to my phone to turn on my mobile data …

While I am working at my desk, my phone is plugged into the charging point some distance from me,

I think this is turned off because my phone is not affected and to save battery, I think my phone automatically turns off the data …

How can I solve the problem?