RS700 Doesn't connect to old devices...why?

Hi all and thanks for taking my question/s into consideration.  I'm looking or some assistance as to why my new R700 Router is AWESOME, but not perfect.  

Backstory - Since the beginning of the pandemic, I had been using an AC1900 router with a CM2050V Modem to get what I thought were pretty hot speeds around my house (90-140mbs).  My setup required I put the router in the basement in order to be in close proximity to a Gigabit Switch that fed the rest of the house wiredly.   Because of the basement setup, WiFi coverage wasn't the best, so I had a EX7000 Extender connected via Powerline to the router which did a really consistent job of coverage and throughput.   Well...Xfi(Ye Cable Co Whom Shall Not be Named) OR my router really started giving me fits recently in the sense that it would totally drop service both wired and wirelessly, and I was going out of town in a day or two while the fam needed to continue working/home-working from home in my absence.  I researched what was in the area and settled on the RS700 as total overkill, but futureproofing at the same time.   I was familiar with the web-interface adn setup went smoother than I thought.  However, one aspect, for simplicity sake might be bugging me...help me out here:   The SmartConnect feature is awesome and limiting at the same time.   It provides seemless connection, but I THINK the security features are too advanced for all devices.   Now I have SUPER HOT connectivity both wired and wireless (bouncing at 610, and often 500's settling at 470mbs all over the house) [I know, I know...it's not the hottest, but for us, it's amazing!...we'll go with it for now until tweaks can make it better].   Now...I set up the router with a SSID (hidden) and PW the same as the router before expecting every wifi device just to remake the connection to a new central box (that I moved upstairs for better wifi coverage).  Many devices had no issues hooking up, but I have 2 or 3 (An Amazon Kindle, Chamberlain Garage Door Opener, and a couple others I can't think of RN) that won't connect REGARDLESS of how many times I reboot the router and manually set and reset the wifi address on the devices.  

My troubleshooting has led me back to the SmartConnect feature...when it's ON, it blasts all frequencies 2.4, 5 & 6 in all directions, but it limits me to WPA2 + WPA3 security for 2.4 and 5ghz signals and WPA3 for the 6.   I can turn OFF the SmartConnect feature and it allows me to select only WPA2, but I now have to individually set up each signal.   The list of questions:

- Do the older devices not recognize the WPA2 + WPA3 functionality and are therefore being denied access by lack of compatibility?

     - This would derail Backwards Compatibility to Wifi5 in my mind and lessen the Star rating for this Router if that is the case.

- If I undo SmartConnect, can I set each frequency (2.4, 5 & 6) individually, to the SAME SSID (while not broadcasting it) and achieve the same results as SmartConnect advertised? 

      - This seems to undermine the upside of the SmartConnect/multi-band functionality, but if it gets me 100% accessible, it may be my only choice.

Here's the killer...the Chamberlain Garage opener worked initially, right after I set up the RS700.  After a day of xFi(Ye, whom shall not be named) disconnectivity (they were down in our area for about 6 hours), and a reboot (not a reset, mind you) of the RS700, the Chamberlain wouldn't connect anymore.   

Rambling over, questions made...what say you??

I really don't want to have to reconnect the EX7000 Extender just to get a few devices working when I have a new monster router that advertises full backward-compatibility yet doesn't seem to deliver.  Thoughts?  And thanks for my explanatory diatribe!