I use A-XPOL-0002-V3-02 (with 2x N sockets instead of cable) on FritzBox 6850 5G. The transmission speed can only improve if the provider does not have a speed limit that the FritzBox can reach even with the supplied antennas. In the city, it is quite possible that no improvement will be achieved.

Ookla Speedtest:
With a Meta Quest 2 on WiFi 5 of the FritzBox, I had a maximum of 220-250 MBps DL and 75-80 Mbps UL at night (around 130 MBps DL during the day) with the supplied antennas. Until today, DLmax was 392 Mbps at night and ULmax 110 Mbps with Poynting. However, it seems to be some kind of boost peak performance that does not last constantly; but between 250 to 300 Mbps DL are the measurements at night with Ookla speedtest most of the time.

Yes, you can use it but in my opinion it's not necessary to add a Poynting antenna. Personally, I bought it but I haven't used it. If you want I can sell it to you for CHF 150. If you are interested you can contact me at marc.jenni@mac.com