Why does my S9 go from 3g to 1x when I connect to wifi?

[Android Central Question]

When I switch from cell to wifi the phone goes from 3g to 1x. If I turn the wifi off it goes back to 3g. The trouble is my email has to have a cell data connection or it won't sync. and upload new emails or send any emails until I turn the wifi off. ????

 

[lucianus_luciferus]

Try this but not sure if it will work for what you want to achieve.
Settings, connections, more connection settings, download booster.
Or there is another setting in developer options for 'keep mobile data turned on'