The simple answer is yes, you can install your patio heater almost anywhere you like. The only considerations are: Is it stable? Is it safe?
So if you want to attach it to a fence post, or put it on part of a pergola or for one of the larger heaters, to hang it from some sort of garden structure, thatís all fine.
What you would need to consider is how far away it is from the people it's heating.
However, in terms of where it is mounted, as long as it is securely bolted to a stable, solid surface you will have no problems.