GL-AR150 with external antenna

If one does not attach the external antenna to the device, does the original internal antenna still function?

The short answer: NO.

The detailed answer:
when the router is sent out, it is either standard version (internal antenna) or external antenna version. The external antenna version already changed to a standard uFL connector and have the SMA connector on the case. So you just need to attache the external stick to make it work. So you will not have the internal antenna working even you don’t attach the external stick. If you want to use the internal antenna, remove the external antenna from the uFL connector, then solder the internal antenna directly to the radio part. It is not difficult if you have a little skills.