Hello!
We're grateful for your participation in the Apple Support Community. I'm sorry to hear about the Wi-Fi connectivity issues with your brand new iPhone 15 Pro. To address this, try restarting your iPhone, forgetting and reconnecting to the Wi-Fi network, and resetting network settings if needed. Ensure your iPhone has the latest iOS version and toggle Wi-Fi Assist in cellular settings. If the problem persists, reboot your Wi-Fi router and consider contacting Apple Support Tel.: 0800 6645 451 (DE) or more tailored assistance. Be sure to note any specific error messages or patterns of disconnection for better troubleshooting. I hope one of these steps resolves the issue.
For those who prefer English, we have an exclusive community available at discussions.apple.com. As your post is in English, we cordially encourage you to join our English-speaking community there.
Thank you!