How do enable or put an APN on my Samsung galaxy s5 to download multimedia files?

So I recently got my brother's galaxy s5 and when doing so, the worker at cricket took out the Verizon sim on the phone and put in instead the cricket sim card. The only problem i have is that I can't open multimedia messages I get on the app messages. It just shows a download button on the message and when I click it, I wait a while and it does nothing. I looked up online to fix it and when I followed the steps, I saw my APN was blank. It had a faint/ whitened option called Verizon LTE that I couldn't select. I tried setting to default, the APN but it already was in default

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