Can't connect to mobile Internet?

[Bara_no_Uta]

I just got my phone on Sunday, and when I did the Internet was working just fine. Then I got home and connected to my home Internet so stuff wouldn't count towards my data usage. Then, today, when I was not at home, it kept searching for other wi-fi networks to connect to instead of connecting to Verizon's. I think this may just be a matter of needing to switch it back to 4G, because I don't see that on the top of the screen anymore, but I don't know how.
Any help is appreciated.