Hi Luis,
The way I would go about it is to use one of the many Dynamic DNS services, there are plenty of example MikroTik scripts out there tuned for each Dynamic DNS service. Just ensure the IP interface that the script is using is the DSL one. You may need to use some packet mangling and routing rules to force this. The scripts I have used in the past assume only one connection to the Internet, but it wouldn't be hard with a bit of patience to modify the scripts to work with multiple connections.
This will then give you a fixed DNS hostname for your DSL link that you can add to Observium.
If your DSL IP is static, then depending on your ISP they may allow a DNS PTR record to be added. Some ISPs will do this, many won't, so the Dynamic DNS approach is the safest method.
Regards
Chris Macneill CMIT Consulting