The APN changes depending on your provider, so it’s hard to give you a specific answer without knowing in which country your data plan is based from.
From my experience, you can use Google to find your APN very easily by searching “orange APN internet”, Google should give you an answer in your language that applies to the country you’re in, in my case it shows me the Spanish APN for Orange which is the following
Nombre: Orange Internet Móvil.
APN: orangeworld.
Nombre de usuario: orange.
Contraseña: orange.
MCC: 214.
MNC: 03.
Tipo de autenticación: PAP.
Tipo de APN: default
Once you have that information, you go to your phone settings, “Network and Internet” > “Mobile Network” > “Advanced” > “Access Point Names” (at the very bottom of the list)
Once you open the APN list, you should be able to create a new entry and fill the details you got above from Google, don’t forget to save and after that the APN should be properly set and you should see 4G available again.
If the issue persists after setting the APN to your phone’s provider, it might be that you are experiencing a different issue than the one described here.
Hope it helps !