Login in my Wi-Fi router after factory reset

hai sir/ma'am,
help me to login in my Wi-Fi router,
I factory reset my Wi-Fi router because, every single minute WAN is disconnected and then, whenever WAN is connected it's slow down to speed 55 to 75 KBPS (I tested Wi-Fi speed) and, my modem speed is 2.5 MBPS., Therefore, I factory reset my Wi-Fi router.
Help me to login in my Wi-Fi router.
(I go to my Wi-Fi router login website, but it is not opening)