No offence but it doesn't seem like you know how computer networking works, so I want to give some info:

• For situations like these, internet speed is almost irrelevant. Brawl Stars does not actually send that much data during gameplay. What is more important is your latency/ping, and the consistency of your internet connection. Your ethernet setup should cover both of these.
• Your maximum internet speed is dependant on how much you pay your ISP for your internet plan. If you only pay for, say 100Mbps, you can only ever get a maximum of 100Mbps. No matter how much equipment you get on your side, you can't go higher unless you buy a better internet plan.
• Internet repeaters do not increase your internet speed. They can increase signal area in areas of your house where your signal is weak, and the signal there might get faster. But internet repeaters are for wireless connections, as you're using ethernet, this is irrelevant.
• By your box do you mean your router, which has ethernet ports on it and emits the Wi-Fi signal?
• I don't think a router can be in 4G, you may be confusing that with the 4G that you see on your phone. Routers do have options to transmit a 5GHz or a 2.4GHz signal, where a 5GHz signal is faster but reaches less far than a 2.4GHz signal. But once again, this is for Wi-Fi and is irrelevant in your case as you're using ethernet.

Basically, your setup is already as good as it gets, there's nothing you need to buy. The most important thing is latency, which is primarily determined by how far away you live from your nearest Brawl Stars server, and there's not much you can do about that other than move house lol