Router stops working after accessing it on IP

We have 10 cameras connected to an XVR. This is connect to a Mercusys router, and we want to send this signal to a webpage using No-IP DDNS.. We need to configure this on the router but we have this problem: This router doesn't have a default/factory password and so the first time we visit the router's IP we are asked to set a new password. So, we set a password, but then the internet stops working. The internet only works again after we hard-reset the router, and that, of course, erases the password. So we're stuck in this loop: Create a password, internet stops working, reset the device, no

Question by Maxim Baurov on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.