I have an s21 ultra and I do not have the option to select 5G under network mode.. Wife has the same phone and she does.. Swapped around sim cards and I still do not have the 5G network mode.. APN's are identical with same service provider.
Are both phones the same version either locked or unlocked. For instance my Verizon locked from Verizon phone only allows me to select auto or manual network (carrier) and not band.
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S4, Galaxy Watch3 (VZW)