buy prepaid t mobile card to travel to USA

Thanks for reaching out! Sounds like we have three questions. How can you buy a T-Mobile SIM card? Will your phone work with a T-Mobile SIM card while you are in the US? Can you use that phone with your original carrier SIM card on our network through roaming?

T-Mobile SIM cards are available to purchase through many retailers. I don't think Walmart and Target are international so Amazon may be the best option. Just make sure you purchase a United States version and not the German version. 

Network compatibility can be confusing and changes as technology evolves. Hopefully I can shed a little light on things to make it easier. Cell towers broadcast frequencies and your phone has to have the right antenna to pick up what is broadcast. When your phone picks up the frequency, the software in your phone has to be able to process it to actually send and receive calls and text messages.

1. I found the specs for the oppo A54S online and it should be able to pick up 5 of the frequencies that T-Mobile uses. 
2.  What we can't verify is if your phone is capable of VoLTE. If it does have VoLTE, then you can get the T-Mobile SIM card and use your oppo A54S while you travel. If it doesn't, then that phone would not get service with the T-Mobile SIM card. 

If your cellphone provider and T-Mobile have a roaming agreement, then it is likely that you will get some kind of service while in the US. It is hard to say what will and won't work because the phone is not built with the US network top of mind. You will most likely be able to send and receive text messages and get some data. If the phone is not VoLTE capable, then calls may not go through at all with either SIM card. 

It is a bit of a long answer but hopefully this helps.