The very example I provided comes with an mPCI-e slot to install a WiFi card of your choosing.
Also they have SIM card slots so you can install a data SIM card and set-up a fallback configuration that switches to it if your landline internet connection goes down.
Yeah, the case with antennas is a good point - when I decided to concentrate various things in a Mini-PC in my living room (TV-Box, Router and so on) I actually looked into these router Mini-PCs as an option and the biggest problem was the lack of a proper antenna, so I ended up going with a generic Mini-PC and leaving out the router functionality which remains done by my old router (which is quite decent, just a bit outdated).
Mind you, this one also wouldn’t work for me because I’m using 4 Ethernet ports (1 for the external connection and 3 internally) whilst this one only has 2 (a weird choice for a router).
IMHO, this isn’t really better than just getting an SBC with 2 Ethernet ports and WiFi and put it in a box with an antenna), a setup which suffers from exactly the same problem as this one: not enough Ethernet ports.