You are effectively asking if a spellcast that is granted by a class feature that is not the Spellcasting feature is subject to the "only one non-cantrip spell may be cast per turn rule."
The answer is that it is and therefore you may not.
The rule for casting a bonus action spell states:
A spell cast with a bonus action is especially swift. [...] You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
This is a blanket statement that doesn't care about the source of the spell. It does not matter if the spell is cast as a result of a magical item, the slots granted by the Spellcasting feature, or some other class/race feature.
If you, by whatever means, cast a bonus action spell, you may only cast a cantrip with your action.
There is a caveat to this.
Sometimes there are class features that say they produce an effect without mentioning the phrase "you cast" and conclude with the phrase "as if you were concentrating on a spell." So long as the words "you cast" are not used, the ability is not a spellcast.
For example, the Archfey Warlock's Dark Delirium ability:
[...] as an action, choose a creature that you can see within 60 feet of you. It must make a Wisdom saving throw against your warlock spell save DC. On a failed save, it is charmed or frightened by you (your choice) for 1 minute or until your concentration is broken (as if you are concentrating on a spell).
Though it functions very similar to one, this is not a spell because the ability does not say that the Warlock is casting anything. It merely borrows the same concentration mechanic from those kinds of spells to serve as a possible way for the effect to end.
You have to be careful not to get lured into thinking that the phrase "as if you were concentrating on a spell" means it's not a spell in all cases. It's important to read the full description of the ability. For example, your Mantle of Majesty also includes the phrase "you cast".
It does this because, in addition to casting Command, the ability tacks on the added benefit "you take on an appearance of unearthly beauty for 1 minute" and it's clarifying that this extra benefit also ends when you lose concentration.
Another thing you might see: some abilities/features use the phrase "as if you had cast" such as the magical item Ventilating Lungs and its gust of wind ability. This phrase indicates that you are not casting a spell but achieving the same effect as if you were.