The thing is, direct bans create pushback, especially at that age. Instead of telling her "stop using TikTok," you could start from what she's actually feeling - most teens scroll because they're looking for a break, social connection, or just to decompress. Try having a conversation where you ask what she really likes about it, without judgment. Like "I've noticed you spend a lot of time on it, what is it about it that you like so much?" and actually listen to her answer. That builds trust instead of turning it into a confrontation.
Then involve her in finding a solution instead of just imposing a rule. You could think through the real problems together - homework piling up, staying up late - and she'll probably come up with limits herself. Like "if I do my homework first, I can watch for 30 minutes" or "I'll leave my phone outside an hour before bed." When it's her idea, she'll stick to it way more easily. You could also set an example by limiting your own phone use around her - teens pick up on that right away.
Timing matters a lot too. Don't have a serious talk right after she's been scrolling, or when she's already on edge. Wait for a neutral moment, maybe while you're doing something together that she enjoys. And be ready to adjust as things change - what works at 15 might be different later on. The whole point is for her to learn to regulate herself rather than just follow your rules.