I posted this on the wrong forum on Thursday when I found this out and just realized it! So reposting here because 5G is only available on the newer phones and I meant to post here originally anyway 😊
TMobile has just released an update about their 5G and issues with SIM cards. They have found that customers whose SIM cards have the letters TM plus 4 numbers are not 5G compatible. The new compatible SIM cards have R15 on them and are compatible. If you have a non compatible SIM card and have a 5G phone, please reach out to TMobile for a new SIM card.
I found that I actually did need to replace my SIM card as it was one of the old ones, which I knew it was but everytime I called TMobile and asked they told me it was a good one. I went on Thursday and replaced my SIM with the new one and my phone has been working much better, switching between 5G and LTE without a problem.