expert in local server POS system, router, access point, ipad, local ip address

i have set up local server pos system, php code. supposedly, my pos system is not related to internet connection at all. but sometimes when there is no internet connection, my connection from ipad/mobile to local server through wifi ip address is badly affected. my clients use mobile/ipad to connect local server system through router/access point with wifi local ip address set up. when there is internet connection, the connection from mobile to local server through wifi local ip address [login to view URL] is good. But, when there is no internet connection, the connection is extremely bad or failed totally. This kind of cases sometimes happen, sometimes not. i suspect it is due to cache stored inside router. i need someone good at POS system who is familier with using this wifi router access point linked to local server.