APN is grey out on my Android phone

[cjsy2001]

I would like to buy Japan SIM card when travel in Japan. The SIM card requests add new APN, but when I look at my APN setting, it is grey out. Does anyone know how to add new APN for new SIM card in this case? Thanks

[cjsy2001]

My phone is OnePlus 7 pro, Android 10 OS. 

[Luddite]

What make/model is your phone? iPhones do not allow manual adjustment of most APN settings.