You're in the same situation as thousands of parents right now, and the good news is it's not irreversible. Six hours a day on TikTok at 14 means the app has basically got him hooked, and yanking his phone away suddenly just creates conflict without solving the real problem - which is the addiction. The "crazy scenes" you're describing are symptoms of actual addiction, like withdrawal. If you do it that way, he ends up frustrated and you end up in constant war. It doesn't work.
What works better (and I say this because I've seen plenty of parents do it well) is getting him involved in the decision, not imposing it on him. Sit down with him when things are calm, not while he's on TikTok, and have a serious talk. Don't judge, just observe together: his grades have dropped, maybe he's not sleeping well, he gets cranky when he doesn't have his phone. Ask him what he thinks - does he reckon 6 hours a day is okay? He might say "yeah, I need it" - then figure out together why: what attracts him to it so much? In the meantime, suggest a limit you both agree on, not something you're forcing. Maybe 2-3 hours a day, but he decides on it with you. Some apps have parental controls that he can see, not hidden - make it look like an agreement between you two.
Also: taking the phone away at the table is essential, but give it back after dinner without making a fuss if he's sticking to the deal. Replace at least some of those 6 hours with stuff he actually wants to do (football, gaming on console, something). And yeah, his grades will only improve if he studies more, but it often happens pretty naturally once the phone isn't always there.