Use:
Internet connection is down. All traffic goes to the router that responds with a simple http page that tells users; “The internet is currently unavailable do to a service interruption”
Once the ISP starts working again… traffic goes back to normal.
I am sure something similar must have been done before.
You cannot redirect https - the client expects a certificate for a particular fqdn and wouldn’t accept a certificate for another one, so it would not be worth it to implement this functionality. The only web page you can display is the one associated to the hotspot functionality. There have been similar topics here in the past and the Mikrotik staff response to requests for adding webserver functionality has always been a clear and loud no.
Install in LAN NanoPi NEO or similar platform with www server.
Or use a device, much better suited to hotspot functionality with above-average requirements: openwrt-based device.
There we (can) do such tricks. And a few more.
But your requirement impossible to fulfill cleanly for https, of course.
In best case, you reach the redirected page after some browser warnings regarding “Invalid Certificate”.
Depends upon browser.
I was thinking… Raspberry Pi will serve up that one page.