NIghthawk C7000v2 Changed IP address, can no longer log in to router

> Model: C7000|Nighthawk - AC1900 WiFi Cable Modem Router

   C7000[v1] or C7000v2?  Look for "Model" on the product label.

> I had to change the IP address of the router because of conflicting
> addresses in this house. [...]

   Eh?  "had to"?  Why, exactly?

   What, exactly, do you think is "conflicting" with what, exactly?
Hint: There's no fundamental limit to how many routers "in this house"
can use the same IP address, so long as you don't connect them to each
other.  And if you do connect them to each other, then simply changing
an IP address on one of them won't solve all the problems which you will
have created.

> [...] (Other tenants are using a different [bad] internet provider
> [...]

   Which ISP is the bad one, and which is the good one?  And why does
what "Other tenants" do affect you?

> What can I do?

   Start again.  Reset the C7000[vX].  Describe what you're trying to
do, and why.  Then, if you're experiencing some problem or other,
describe that problem, rather than launching into some scheme to
implement some "solution" ("changed it to 192.168.7.1"), which may have
little or nothing to do with your actual problem.

> [...] I changed it to 192.168.7.1 but after I did that and the router
> rebooted, I can no longer [...]

   If you change the LAN IP address of your router to use a different
subnet, then you might want to restart your client devices, too, so that
they'll be sure to get new/compatible IP addresses, too.

   But if you have multiple routers (DHCP servers), using different
subnets, sharing the same cables, then you can expect a bunch of
problems because any client device will be broadcasting its DHCP
requests to multiple DHCP servers, and taking the first response it
gets.  Which could come from any of the routers (DHCP servers).

> [...] I can no longer log in to the router [...]

   As usual, showing actual actions (commands) with their actual results
(error messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

   If you want to try to puzzle out what's going wrong now, then you
might want to look at everyone's IP address, and see what makes sense
and what doesn't.  But I suspect that when you stop connecting multiple
routers together, all those problems will disappear, so careful analysis
of the current mess might be of little long-term value.