It has antenna they are just built into the case. Although you would have to really dig for details there is not a huge difference in wifi converge between devices. Most devices transmit very close to the legal allowed maximum. Devices can only have 30db of transmission power. You could have a 20db radio and a 10db antenna or you can have a 24db radio and a 6db antenna. Either way you get the same power output....ignoring illegal aftermarket antennas.
The router is only 1/2 your problem the end devices generally do not put out the maximum power because they run on batteries and/or they have very tiny antenna to be portable.
Most times you use a separate modem/router for software features. Most combo devices are pretty...