With render it might be worth isolating the bracket from the
surface...

For small items "Rigifix" might work, they use a 10-12mm hole with a
plastic plug, then an allen key driven sleeve, then a standard metric
fastener (bolt) of M6-M8 can be screwed in preferably with some thread
lock compound.

For large items I would 25-38mm core drill via cordless through the
render to brick work. Cut some acetal round bar longer than the render
is thick, width chosen to fit within the core drilled holes and
drilled to take a stainless screw. Drill the brickwork for Fischer
SX10, fit together with stainless coach bolts. Silicone around the
render-spacer interface.

For a chimney, you run into several problems.
Insufficient mass above the fixing. Very tall chimney do flex with a
large aerial & gust loadings, or they crack resulting in the bracket
pulling a brick out of the chimney (brick shaped hole in stack
opposite, aerial & brick on roof). A crack in a very tall chimney will
propogate until the thing if lucky starts to twist making it obvious -
or simply plunge ground wards.

So perhaps find a wall.