Faulty phone

About a month or two ago, I purchased an Oppo A40 phone from Truworths Liberty Promenade account which was partnered with MTN. When we purchased the phone, we installed the needed apps and updated the phone. I was told by the sales consultant that we need to allow the battery to run out then charge the phone in which we did. I installed EVOD and Crunchyroll where I can watch content and the phone was extremely lagging. I then resumed with uninstalling the apps and the lagging continued. The phone has a 45w charger yet it takes close to an hour to get the phone fully charged. The phone was freezing in our hand and then within 7 days we took the phone to have the phone checked out and reviewed. They said they have to send the phone to MTN to have the phone investigated. I waited for almost 3 weeks in which that was the agreement. When the phone came back the phone was exactly the same as when it was sent away. Still doing the same. I spoke to the manager of Truworths and she advised that we need to wait for the cellphone sales person to resend the phone to MTN in which we didn't have a problem, however I received an SMS saying the phone is fixed and we need to wait for 3 days to collect the phone. This goes against the policy of truworths and MTN as I read the terms and conditions where the phone will be swapped for a new device within 7 days if the phone has out of box failure issues. I have kids with 2 Oppo A18 phones and those phones are far superior in speed than the A40 which is suppose to be a far superior phone. This is unacceptable behaviour as I already made my first payment for the phone but haven't had use of the phone.