Remote shutter release
Canon's 90D is my first DSLR to NOT use their N3 remote shutter connector.
It instead uses so-called E3 or N3, a 2.3mm (3/32") TRS phone connector.
Neither inline N3 nor mini stereo female jacks are readily available,
and the only converter cable found was more expensive than an E3 remote,
making a single remote for both not economic.
However, I lucked into 4 original (but used) Canon RS60-E3 remotes
for less than new clone price, with many clone reviews complaining
about lack of tactile feedback between focus and shutter press.
Canon's RS60-E3 also turns out usefully smaller than their TC-80N3 or many clones:
Two notches in RS60-E3 bodies are seemingly for cable storage:
My RS60-E3s lacked instruction sheets,
also not found on Canon's website.
No YouTube review videos seemed to get this simple device right,
either incorrectly suggesting that it uses a battery, works with all Canon DSLRs,
or is unnecessary for cameras with touch screens.
In fact, many times I want neither to glance away from the viewfinder
nor touch the LCD for exposures when using Live View.
Canon has seemingly discontinued this remote, preferring to sell either IR or RF remotes.
While I occasionally use a tiny Bluetooth remote to trigger iPhone's camera,
I do not care to carry around an easily misplaced RF remote pair, both liable to dead batteries,
much less an IR remote that is likely problematic in bright sunlight and blind from many angles.
Since usefully smaller than N3 remotes, I will get a cheap N3 cable to repair the faulty RS60-E3.
Canon's web page for 90D-compatible remotes
lists but shows neither RS-60E3 nor BR-E1.
If they cared, their page might look more like this:
Compatible Remote Switches (Still Images/Movies) (EOS 90D)
For remote control shooting, use Remote Controller RC-6,
Remote Switch RS-60E3, or Wireless Remote Control BR-E1
Connecting the Remote Switch
Once connected to the camera, the switch enables you to shoot remotely over a wired connection.
- Open the terminal cover.
- Connect the plug to the remote control terminal:
Shooting Movies with RS-60E3
- Set the Live View shooting/Movie shooting switch to <>
- press < MENU > and choose the  tab
- press sub-tab 
- Set [ Shutter btn function for movies ] to [ Start/stop mov rec ].
hk.canon more nearly gets it right..