I have read that people are struggling with N900 phones because carriers drop down 3G support and they say they become unusable. I do not understand about these things and have just thought that 3G phone can not use 4G network. But then there are this kind of articles which suggest that you can use 4g netwotk with a 3g sim but it just does not support the 4g speed https://maccablo.com/convert-3g-phone-to-4g-phone/