1. Library routing itself has many users, which may lead to network disconnection. It is suggested to find a few people to try whether this situation still exists. Just like in the morning. Solution: Go to the library when there are few people.
2. Some old network cards can't adapt to the encryption methods of some routers, such as WPA-PSK/WPA2-PSK, which leads to the connection failure of some old network cards. Or some old-fashioned routes use relatively backward encryption methods, such as wep, resulting in reduced bandwidth. Solution: Change the network card yourself or change the route for the school.
3. It is not excluded that special settings have been made in the router, such as bandwidth and p2p restrictions. Solution: Give the library network administrator a beating and let him modify the settings. Or take advantage of the temptation of beauty.
Another method can be tried: check the ip obtained by your friend's computer network card in the wireless network of the library, and then manually set your network card to a similar ip and the same dns server. The yellow exclamation point indicates that the IP address cannot be obtained. You can try to add one manually.