I just found out that oppo cannot connect 4g in the USA or because of AT&T I'm not sure. I can't fix or setting my phone. If I move to another sim card would it be able to connect 4g?
Nope. If it's not on the list, AT&T won't allow it. (and it won't work) The days of just checking bands and tricking sim cards are over ( at least for the forseable future) https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.att.com/ecms/dam/att/consumer/help/pdf/Service-Capabilities-Unlocked-Devices-ATT-Network.pdf&ved=2ahUKEwiX-Jv8oYLtAhXQo1kKHYhHDGcQFjAAegQIARAB&usg=AOvVaw3sCP2kJRUevj0ARWKoc1nJ
Hello @OppoUSA, We can definitely provide you with some more information about your device. The cell phone that you're using is not an AT&T branded device, so unfortunately there is no guarantee that all of the features will work properly. We recommend that you Check the compatibility of your device with the AT&T network, and see if it shows that your cell phone is compatible or not. If the previous link does show that your device is supposed to work on our network, you may need to make sure that you're using a compatible SIM card. Let us know if this helps. Sean, AT&T Community Specialist
Neither will Blackview, Huawei, Honor, Oppo, Sony, Ulefone, Redmi. If it doesn't come in a North American version, with a US warranty, it won't work.