Why is my phone not connecting to 3G or 4G network

Please I need help with my new phone G5plus which was sent to me from abroad (USA). The name is G5 plus ( Blu Studio Mega 2019) and it runs on android 9. I just feel depressed for over 5days now because I have tried everything possible. The issue is that my phone network is only showing 2g network. I even bought a new 4g Airtel Sim yet is still showing 2g network. But on puting the Sim card on my friends phone it will show 4g network. If I change my preferred network type to 3g/4g the network bars disappear entirely. But changing it to 2g/3g/4g the network appears back with a 2g network. Please what do I do, I am not really enjoying the phone cause of slow network. I am Nigeria.