Regarding the weak connection, Perhaps this link of recpommended settings will help. It really is not important unless you have crooks next door.
Recommended settings for Wi-Fi routers and access points - Apple Support
Regarding the poor or no connection
· Reset the router to factory settings (menu or paper clip) – even if all other devices connect OK!
· On iphone Settings, General, Transfer iphone or reset, reset, reset network settings
· If you have a VPN you are strongly recommended to uninstall it and its profile in Settings, General, VPN and device management
Also see Check VPN to resolve problems and connectivity issues
Check VPN (Virtual Private Network) and third-party software to help resolve network connectivity issues - Apple Support