Finding a professor or mentor for a postdoctoral fellowship through an individual posting on ResearchGate, for example, is certainly a strategy. But whether and to what extent it is ultimately successful is an important question that points to the general conditions for the success of a postdoctoral fellowship.
Even if it is possible to establish contact with potential supervisors, it is unclear whether this person is the right one. Just because someone offers me a position or even promises me one, that doesn't mean anything. It is essential that you first clarify for yourself in which academic field and with which research focus you would like to further your academic qualification. Based on this, you should research nationally and internationally to find out which scholarship and exchange programs exist that could be advantageous for your desired academic profile.
Now, based on criteria, a targeted selection must be made among relevant universities, schools and chairs that could potentially or optionally come into question. The following criteria have to be considered in particular: the academic reputation of the host institution, the professional fit with the main supervisor or research unit, the scope, duration and funding of the fellowship, details regarding work and residence permits, etc.
At the end of this selection process, you will see which suitable programs, funding and fellowships remain and whether and to what extent you can meet the required application criteria. Once you have applied and have been shortlisted for the position, it is always important to take a self-critical look at whether the working conditions and supervision offered suit me both professionally and personally.