S4 SCH-i545 Can't send messages on 4g

I recently switched carriers. From Verizon, I went to T-Mobile. As expected, I'm already having issues. I can RECEIVE texts, but cannot send them on 4G LTE (only when I unexpectedly get "H+"). I can also access the internet. I have contacted T-Mobile, but honestly, they don't seem to care about my issue. I already manually changed the APN. The only thing I can think of is that my phone is currently on Lollipop, which is not yet supported by T-Mobile. Could this be the root of all this? Any help would be appreciated.

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