Well, that's not the way it is supposed to work. The Reply-Message attribute is sent to the router (in this case MikroTik) which can do with it whatever it wants to. I'm quite sure MikroTik doesn't take the Reply-Message from RADIUS to display it on a web page (or anywhere at all).
To achieve what you are trying you could for example try the following:
Users having reached the usage limits are given IP addresses from a separate ip pool.
Then create some redirection rule in MikroTik for all requests from this address range to dst port 80 (TCP) to a local web server which is serving the page you want those customers to see (e.g. "your usage limit has been reached"). You would also need to allow DNS traffic from this separate address range, I suppose - otherwise customers won't see you special web page but just get an DNS error.
Sorry, no finished config at hand - you have to fiddle around yourself...