We are encouraged to associate "certainly" sumptuous brands with a higher status.
As others have pointed out, only adjectives modify nouns, and so "certain" is the correct choice to modify "brands".
However, you should also understand the difference in meaning. Despite being in the same family, certain has an additional definition that certainly does not have:
1. used to reply completely or to emphasize something and show that there is no doubt about it.
2. very likely to happen.
1. having no doubt or knowing exactly that something is true, or known to be true, correct, exact, or effective,
2. impossible to avoid or extremely likely,
3. particular but not named or described.
In your example, you use the third meaning of certain to mean "a particular but unspecified set" of sumptuous brands. Since certainly does not have this meaning, you would not use it.
There are certain people who like to answer questions about English.
Certain fair-skinned people should avoid direct sunlight, as they burn easily.
Yes, she has a certain attractiveness, but she's still not a good actress.
Note the difference between the third example sentence and these two:
Yes, she is certainly attractive, but she's still not a good actress.
Yes, it is certain she is attractive, but she's still not a good actress.
Here certain/certainly have the other meaning, "without question".
You should use "certain" if you are speaking about "certain brands that are sumptuous" since only an adjective can modify a noun.
You could use the adverb "certainly" to modify "sumptuous" but the phrase "brands that are certainly sumptuous" doesn't make sense.