My MMs isnt working . Maybe APN issue or? Need help.

[0hmai]

Hello, I recently purchased an unlocked Verizon Lg G3 vs985 phone. I also purchased a AT&T micro sim card to transfer over my service.

The data does work. I can get on and off the internet without Wi-Fi. I can also make and receive calls. But today my friend tried sending me a picture over text message and it keeps saying "message expired or not available".
I don't know if it's because my APN are not right or I need a different sim.
Would a straight talk cdma sim even work on my phone ?
I am not a tech person, so I have no idea how to go about this issue.
Also, a lot of times my 4g lte switches to 3g and the only way to get 4g back again is to go into my setting and change global to gsm or gsm to global.
Any help would be greatly appreciated, thanks .

 

[hang-the-9]

Contact your cell carrier and find out what you need to do to get this working, what you are seeing is one of the issues you can run into when swapping phones between carriers. https/www.straighttalk.com/wps/portal/home/support/apn