Asus uses ASUSWRT which can be decompiled, modified, etc. but as for data removal, once it's gone, it's gone. I can't see anyone in the forensics field willing to give the time of day to perform any forensics work on a SoHo router, so you would be wasting a lof of time. If you have telnet access, you can try checking your backup logs (in the event you didn't know they existed) if you're running the Merlin firmware. The backups are stored at /jffs/syslog.log ... If you are not running Merlin, you can upgrade your router firmware, or configure a syslog server, and have your router send logs there. Common rule of thumb... Change passwords to something extremely strong especially at your main point of entry
"I can't see anyone in the forensics field willing to give the time of day to perform any forensics work on a SoHo router" The cost of going through the motions of performing a forensics/data recovery would be very expensive. Based on the type of router you mentioned, one can infer that it is used in a small business, which likely means, the this business is less likely to fork over thousands of dollars for the analysis, and or recovery. Forensics is not cheap, and it is not a quick process.