The word "simpler" means "more simple", as in "This new product is simpler than the old product".
But I cannot think of a similar word for "more complex": Suppose I want to say that "This new product is more complex than the old product", except replacing "more complex" with just a single word. "Complexer" is not a word I am familiar with.
I did some Googling, and e.g. none of the words here work. I don't mean fancier, denser, higher, etc. I mean "more complex" as in, "contains more moving parts / harder to understand and use".
https://learnenglish.britishcouncil.org/grammar/english-grammar-reference/comparative-superlative-adjectives
You will need to find an adjective that means "complex" but falls in one of the categories there to get the "-er" form of the comparative. Note that "simple" is in the last category where it is up to you ... say either "simpler" or "more simple".