how to force connect iphone to wpa/wpa2 only ruter

Check to see if the Wi-Fi router supports WPA2 Personal, also known as WPA2-PSK-AES, which is a more secure setting than WPA/WPA2 aka WPA/WPA2-PSK-TKIP.

If there is no option for WPA2 Personal, also known as WPA2-PSK-AES, your router is long out of date. Nothing else that you can do at this point other than replace the Wi-Fi router.

Look for a router that supports WPA2/WPA3 since it will not be too much longer before WPA2 goes out of favor.