T-Mobile carrier aggregation problem

I'm having a problem with my 6p using CA - carrier aggregation. Both bands 4 and 12 are available and my phone is only using 12. I flashed an older radio that does not have band 12 and band 4 is very fast data speeds. If I flash the newest radio in the very same location my phone only uses band 12 and I see that band 4 is available using Lte discovery but I get very slow data speeds. My phone is not combining both 4 and 12. Does anyone know a fix and have you ever had this happen to you? I've even flashed factory image with no luck. Thanks

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