Have you put in APN settings, saved them, and selected them to be used? Also, with Android 8.0 sometimes mobile data is turned off by default. Just check and make sure it is toggled to on position.
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Also, in your phone dialer dial 277634#*#, in the menu that shows up, scroll down to band selection, select all bands and save. If this helps let us know so others know what to do as well. Thanks.