Galaxy Note 9 4G issue

Hi there,
I have a branded Galaxy note 9 (Band Lte). I want to use it in Nigeria and I am limited to just one Sim as against the 2 Sims slots it has due to lock mode. The 4G has refused to be switched on as I turned it to 3g and it refused to go back. How can I solve this please, the phone is just a day old with me b4 I noticed these.