You may take a (long) read here.
But generally spoken. You should start off with setting the lowest network type which is 2G (both cards) as only one of both may use 3G/4G while the other is forced to 2G. The assignment is totally up to you.
Both cards may be set to 2G at once, but not to 3G/4G. So the configuration sequence makes the difference here as the setting for the limited card to 2G will be greyed out once 3G/4G is defined.
1st configure your preferred card for calling (any socket), Last configure your data card with 3G/4G.
If you are completely stuck, power down the phone, remove both cards. Have a power cycle. Once switched off again, reinsert both cards and retry after the next boot.