Samsung Galaxy M series (M21, M31, M51…) has Exynos 9611 chipset which supports carrier aggregation, but this feature is missing and not available at this moment.
The carrier aggregation feature lets your 4G mobile connect to more than one mobile tower for better availability and connectivity, and may improve your network options.
The hardware support is there, but the feature is locked due to its affordability.
I checked some of the other models that ship with Exynos 9611 chipset, surprisingly the carrier aggregation feature is not available or locked in all M series mobiles.
This is a vital feature which is being missed out by many brands including Samsung, and we all should raise the issue in respective forum and support page to do not cut such important feature in this age where 5G is being rolled out in the world.
In conclusion, if you are looking for a mobile that has a better internet connectivity options, avoid anything from M series, better to go with A series or higher models.