so there is no other way to install rather than upgrading To RHEL5 onwards for installing mono..
Nope, well, not a sane one anyway. The BEST way is to stay faithfull to the given repo. If it's not in there, you _could_ find it elsewhere. There is a snag: the software may have dependencies the system does not (yet) cater for. Of course, you could install these, but some may need a newer kernel, fair enough, this means compiling a new kernel, and installing it, at the risk of breaking a system that was not meant for that newer kernel.
Granted, I did that once, and got pretty close to a result. I tried to "upgrade" a Redhat 8, until someone (here, as I recall, on this site) advised me to get a newer install, if I wanted to save myself some (educational but maddening) headaches...
Of course, CentOS is similar in many (all?) ways...so, I'd go in that direction...