In old school radio techniques the ideal antenna is one full or a half wave length long.
For the good old 27 Mc the wave length is about 11 meters, resulting in huge antenna's.
For 5 GHz the wave length is 6 cm or about 2 inches, in most cases this result in router antenna's constructed out a single piece of wire of about 6 cm long (put in to a fancy plastic holder as external antenna, the downside is that there are usually connectors or small pieces of coax cable in between the PCB and the antenna wire) or they fabricate antenna's out of metal sheet material soldered directly on the PCB as internal antenna (with the advantage that the antenna can be close to the RF circuit without losses in connectors or a piece of coax cable).
Positioning of the antenna is most important, internal or external. If they position the internal antenna close to an electronic source of noise it won't work well.
Any router with external or internal antenna's shall be placed at a proper location away from sources of disturbance or blocking, besides position also antenna orientation can play a role.