Huawei mate 8 has been extremely poor Wi-Fi reception for a week, what can it be?

All other devices are working properly, only my phone is hanging. Small things I can still do with it but pictures z. B. On Instagram, he can no longer load. He says then I'm not connected to internet. I do not have a shell. Frequency 2.4 GHz. Mobile data works well. Maybe you can give me a hint?

If you still have warranty you can send it there. Otherwise just let fix or buy a new phone