From where is Wi-Fi signal transmitted when replacing 4G router antenna with outdoor antenna?

It's hard to know the antenna design for that router without seeing much more detailed technical information that most vendors don't publish. The two external stick antennas may be WLAN-only (Wi-Fi-only), and the WWAN (4G) antenna may be internal.

Unless you can learn the details of the antenna configuration of your router, your question is not answerable.