wifi on but cant send messages or load apps

I suspect that your pc works on the wifi just fine, just an issue with your phone wifi connection. I also have this issue sometimes. For me it isn't because of proximity to the router, my router speeds, my phone battery, the app server status (although if I were you I would check those things first because they can most definitely affect your connection).

It could also be your router settings. If this is at work then they could have ports blocked for sending messages. Apple's support FAQ states that TCP ports 80, 443, and 5223 are required for Messages.

Ensure those ports are open by logging into your router. Hope this helps!