Settings> Wireless & networks> Mobile networks> Network mode> choose 4G, LTE or automatic network selection. Normally it's automatic. It is possible that you do not have a sufficiently stable 4G, otherwise I do not see: smileysmileywink:
EDIT: oh and you're on the XA2 Ultra dedicated support part.
If you're talking about XA2 (you wrote A2) it's here