I know this is a old tread but

Get a bluetooth obd2 plug for under the dash and get a app from playstore called Carista.
It allows you to change a lot of options not allowed from the nav screen. Including disabling seat belt. Cost 20 but worth it imo.

Lots of discussion about Carista around here and Carista in a sponsor and active member of this site.

Yup
Carista can activate "Rear hatch opening assistance", which means having it open by itself when you press the button.

Please let us know if you have any questions or run into issues. See my signature for more info on the app and hardware adapter. Available for iOS and Android.

Can Carista program smart keys, register ECUs (like the remote starter) and program TPMS tire sensors?

We don't have the options to add/ program smart keys or register ECUs, but we are working on the TPMS. So far we can read the IDs of the sensors and we are working on adding the option to save new ones.

I picked up the wireless OB2 adapter and purchased the Carista sw, works well and worth the money. Check the website it also works for a number of other car manufacturers

I don't know what year your RX350 is, but I have a 2013 and thought I had the same problem when I first got it. The dealer showed me a button behind the glove box cover on the left hand side that controls whether you can operate your rear lift gate with your remote control. Open your glove box and look on the flange for the button (see picture).

My rear lift gate on my July, 2010 built RX350 was not opening with assistance, just manually.

I hooked it up to Techstream and found the setting to change. It is contained under the menu setting for "PSD and PBD Operation".

Works like a dream.

On my '15,the rear hatch opens and lifts using the button under above the license plate.

The thread discusses some owners that did not have lift gate assist enabled in settings. Techstream allowed me to turn it on without a dealer visit and dealer charges!

Techstream also allowed me to program a Smart Key Card and turn off seatbelt chimes and change other settings.

Hi, this is my first time using this site! I just purchased a 2010 RX350 and am experiencing the same issue...tailgate will not open itself via the release above the license plate, just unlocks the tailgate. I bought the Carista adapter and downloaded the app but I do not see the feature in the app. I've seen screenshots others have posted, but for some reason it is not showing in mine. And yes I do have the power liftgate haha. Any help?

Does it open using the key FOB button? This could be a symptom of weak lift supports.

First make sure you are pushing the right button above the tag!
There are two buttons covered in black rubber. The small more pointed spot under the rubber cover is the LOCK button. The wider button to the left of the LOCK button is the power door open button. If that button is not working it might be two things. It will only work without the FOB nearby if the doors are already unlocked. You can press the button to open the tailgate without the FOB nearby if the door are unlocked. If the doors are locked you must be carrying the FOB with you, but you shouldn't have to take it out of your pocket or purse. You will also have to hold the button for about 2 seconds to start the door opening. You will hear the warning beep when it opens or closes. When the tailgate opens the rest of the doors will remain locked. The rear tailgate open button is active by default from the factory, but can be disabled by the dealership. They should not have disabled it without telling you, but if you purchased the car used it may have been re-programmed to disable that feature. It is easy for the dealership to set the tailgate open mode with their SCAN tool. Us mortals can't do it. Lexus chose to not make that function one of the user configurable items in the various menus. Why, I don't know. It's not that special! Once you get the tailgate button working remember you don't have to wait for it to get all the way open to stop it. If you press the button again after it is opening it will stop, right where it is.

On our 2013 RX450H rear hatch.......just press the button by the license plate and hold it for like a second or two and then there is a beep and let go. The rear hatch will power open by itself. I wished this were true in our 2013 LX570! !!!

I just realized this was an old post!!!