Short Mountains and stubby aliens
Let's first establish the general elevation of the mountains on your planet. Most things, due to the high gravity, will probably be shorter here, both the lifeforms and the mountains.
From the physics exchange on the effects of gravity:
In general, the bigger something is, the less it can deviate from
having a smooth surface. The reason is tall mountains will be crushed
under their own weight, and valleys will not be able to support their
Also relevant is the angle of repose, where materials have a specific angle at which they can naturally rest. With higher gravity comes a smaller angle of repose, due to the materials forming the mountain naturally being 'pulled down'. This means that mountains will be naturally less sloped, and shorter.
Finally, the cold climate tends towards easier erosion, meaning any high mountains will be quickly eroded down by the colder than usual global temperature and weather.
Relation between gravity, temperature and atmosphere:
The thickness of a planet's atmosphere depends on the planet's gravity
and the temperature of the atmosphere. A planet with weaker gravity
does not have as strong a hold on the molecules that make up its
atmosphere as a planet with stronger gravity. The gas molecules will
be more likely to escape the planet's gravity. If the atmosphere is
cool enough, then the gas molecules will not be moving fast enough to
escape the planet's gravity.
Another excerpt about Mars' atmosphere height compared to Earth's:
The scale height of the atmosphere is about 10.8 km, which is higher
than Earth’s (6 km) because the surface gravity of Mars is only about
38% of Earth’s, an effect offset by both the lower temperature and 50%
higher average molecular weight of the atmosphere of Mars.
From the above articles we can garner that higher gravity and colder temperature means, in general, a slightly shorter atmosphere height (that is to say, the atmosphere here is shorter and more condensed compared to that of Earth's).
Molecular weight of the atmosphere here seems to be different than that of Earth's due to only 2.2atms of pressure despite 3.6g's of pressure, but despite this the high gravity and cold temperature will contribute to a much shorter atmosphere scale height. We'll ignore the effects of having/not having a magnetic field on the atmosphere loss to space as well.
Highland Height Estimate
Thus, using the conclusions from the two sections above, we know that (1) the mountains are shorter on your planet, and (2) so is the atmosphere.
This means that most likely, the highlands on your mountain will be significantly less tall than the ones on Earth.
In general, the highlands on Earth are roughly considered highlands at 500m and above, some reaching even 2500m.
Thus, considering both the lower mountains and shorter atmosphere height of your planet, the highlands will be shorter in height than that of Earth. A reasonable estimate would probably be anything 250m and above.