Optimize UniFi Network Speed and Resolve UniFi network performance

I'm looking for an expert to resolve slow internet speeds on my UniFi network. This issue affects both computers and mobile devices, and occurs throughout the entire building, not just near the router.

Key Requirements:
- Diagnose and fix the cause of slow speeds
- Ensure stable and fast internet across all devices and locations
- resolve an issue with websites that do not open from my PC (connected by cable) or my mobile when connected to WiFi; while these websites can be open normally on my mobile using 4G

Ideal Skills and Experience:
- Strong background in networking, especially with UniFi systems
- Experience in troubleshooting and optimizing Wi-Fi networks
- Ability to provide a comprehensive solution for improved performance

My UniFi configuration is:
- DMSE connected to my ISP router
- USW-24-G2 switch connected to DMSE
- WiFi mesh include 10 U6+ and 2 U6-LR
- one U6+ and one U6-LR are connected directly to DMSE
- 2 U6+ are connected to USW-24-G2 switch
- the remaining WiFi devices are wirelessly connected into the WiFi mesh

other requirements:
- create vlan-unknown to have by default any new device connected to it
- create private vlan-home for home devices restricted by MAC address
- create vlan-IoT for IoT devices
- create vlan-guest for devices connected to guest network
- educate me on the setup for future changes by myself