Well, maybe the gateway is also required if we choose static ip, since it is in ap mode?

of cause not, as it don't have two wan port maybe you can refer to r480t+...

you can find it on the label at the bottom of the device

check the dns server, such as, change to google dns server 8.8.8.8 ?

access point mode cannot config that, it is enabled by default since... assume a layer 2 topology, wan side = lan side = normal wireless if the guest wireless want to access wan side, means access...

flash dd-wrt first, then flash offical firmware? not sure if it works on your model