If you're a brand growing an audience on Tiktok then you should definitely be posting on reels too, but some find that reposting their Tiktoks onto reels, just isn't getting them enough engagement.
One trick that has proven to work for lots, is to make use of the in-built text/sticker features on Instagram. Doing this will push your video much further than posting a normal video with text from TikTok. So when you are making video content for TikTok, save a version with no text, and redo the text inside of Instagram's reels system.
Make use of polls, stickers, countdowns, and all that Instagram has to offer. Instagram will most of the time favour content that uses these things!
If you anyone has any other questions about Instagram or TikTok, feel free to ask!