The AC1750 aka Archer C7 has 6 antennas, 3 for each frequency band. The external antennas of the Archer C7 are for 5 GHz only, the antennas for 2.4 GHz are inside the router's case and cannot be detached. Since TL-ANT2408C are antennas for 2.4 GHz, you cannot use them to replace the external antennas.

Actually, the standard 5 GHz antennas of the Archer are pretty good and fit to the WiFi adapter's electrical specifications, so I wouldn't replace them. If you experience bad link quality between devices and the Archer, choose a better mounting position of the WiFi router instead of trying to compensate using external antennas. For best results in the 5 GHz band there should be almost no obstacles (e.g. free line of sight) between sender and receiver.