Set static IP on LAN for OpenWrt, but cannot access the device

I bought a device with openWRT pre-installed. I am using a Huawei HG8245Q2 as modem.

I set did all the WLAN configurations on OpenWRT and had disabled the DHCP to use the DHCP of the modem. All went fine, both devices were connected on their LAN ports and I could access everything ( openWRT was on port

Then, I noticed that the ad-block was not working and I decided to try to use it on the Huwaei's LAN to openWRT WAN port. Before changing the ports, I set the LAN interface on openWRT to static IP and set the MAC address to get a static IP from Huwaei.

Now, the internet is working but I cannot access the OpenWRT or change anything there. I used a port scanner to see if I could find a device with port 22 open, but no luck. I cannot find the openWRT device on the list of clients on DHCP either.

How can I access the device?

See also: IPv6 ULA / LLA and ARP / NDP cache.