Does your network service provider support 2G in your location? Check your provider's website to see if they still have a 2G network and if it is in your location.
What happens if you select 2G and then do a manual search for other mobile networks? Do any show up?
A lot of service providers have shutdown their 2G network and the frequency has been reallocated to the 4G/LTE range.
Do you know if it has ever worked? Also is the phone still under warranty?
You will have to find the problem by a process of elimination. You already know that the SIM card is not the problem. Next is to determine whether it is a hardware or software problem.
I suggest that you backup the phone using the phone's back up feature, ensure that the battery is fully charged and then perform a factory reset on the phone, using the phone's factory restore feature.
Be aware the a factory reset will erase all your user data and downloaded apps. It will restore the phone to a factory default condition. It will be like when you first bought it. That is why it is important to do a backup first before the reset.
Once the phone has been restored to its' factory default condition check to see if the 2G now works correctly.
If it doesn't then there is a problem most likely with the hardware of the phone and if it is still under the manufacturer's 12 month warranty, consult the warranty documentation that came with the phone (or look online if you can't find it) to see what you have to do to make a claim for a warranty repair or replacement.
If the2G works after the reset, then restore the phone from the backup you made earlier using the phone's restore feature.
Once the phone has been restored, check to see if the 2G is still working correctly.
If it is then the reset has corrected the problem.
If it is again not working correctly a downloaded app may be causing the problem. The trick is to find out which one. You need to uninstall each downloaded app one by one and test the2G after each uninstall to find the one which is causing the problem.