I’m unable to make Nextcloud accessible after installing OpenWrt on TP-Link Archer A6 router. I just updated the router firmware to OpenWrt from stock firmware and now 192.168.0.162 (NC address) is no longer accessible. Router IP is set to default 192.168.1.1. I’ve gone through all the router settings trying to spot what change to make, but can’t get Nextcloud accessible. I can no longer access NC system settings. OpenWrt assigned a DCHPv6 address and DUID, but I can’t seem to assign it its previous IPv4 address and get it to work. Am I missing something?
OpenWrt is not (well, barely) involved in this, so it can't really break it. The only things OpenWrt does in this usage scenario of two peers in the same LAN segment talking to each other are:
DHCP
the client(s) request an IPv4/ IPv6 address from the router, which are (typically) granted, either from the dynamic DHCP pool or relying on a static DHCP lease (recommended)
unless manually configured, the dynamically assigned DHCP lease might very well differ from the one previously handed out by the stock firmware
DNS resolution of internal hosts
this shouldn't break anything, as long as you use IP(v4) addresses to connect to your nextcloud instance.
Debugging should now start with the nextcloud host, which IP does it get, can you reach the outside ('internet') from the host, can you ping it and ssh in from your client. If that works, great - then your issues would be 'only' with the nextcloud (or the underlying LAMPP stack) configuration, e.g. things like hardcoded (whitelisted) IP ranges, DNS names, etc.
One imaginable cause of problems would be the private IP range having changed (192.168.0.x to 192.168.1.x), which could cause issues with static interface configurations of your LAN hosts or overly tight address bindings of your httpd/ nc configuration.
It was something related to that last point. I fixed the issue by adding under Network-Interfaces-LAN-IPv4 the IP range to correspond to the IP assigned previously to the NC instance.