No Internet when another Android device is connected to router
by reserving I guess uboat55 means to .. suggest the use of static IP address. you can configure your router to work with static IP address instead of a dhcp service.which I also feel should do the trick .

configure your router with fixed IP address and latter use that IP address.

or you can even try to assign IP address directly to ur device A. without changing any thing on the router part.

just remember the IP address range. 
eg 
192.168.1xx.xxx to 192.168.255.254 

and chose any one IP address within that range .

and remember the default subnet I.e 255.255.xxx.xxx or something similar 

and the router IP address and use this on ur mobile .