SUCCESS! Just got it up and running, a bit of a run around and I can save you guys some time:
---Primary DIR-1750 (Your existing one)---
-Backup configs...because you know...sunspots and what not.
-Update firmware to June 2021 Ver 1.11
-Enable the MESH mode from the web console Settings tab then Wireless, NOT THE DLINK APP (Doing it in the app caused me a weird settings loop that made me have to reset the router to get it back up. Good thing I backed up my configs.
---New DIR-1750 (Your new or refurb purchase)---
-Do the initial reset/setup to get into the web console, don't bother with any settings change yet.
-Update firmware to June 2021 Ver 1.11
-Log back into the web console (again NOT THE DLINK APP) Go to settings tab then Operation mode and set it to EXTENDER MODE.
After a few minutes it will reboot as Mesh range extender.
-Now its just as simple as the push button setup offered in rodrigovmd post above. https://www.youtube.com/watch?v=d0LFY0kpxTg
Your D-Link phone app will read both devices and you can see them in the web console as well. But for some reason the new extender is read as a client device and not listed in the extender category of the web console.
Have tested speeds and found no reduction for the last 24 hours nor has the extender or primary gone down. So crossing fingers this is good to go for awhile!
Let me know if you guys have any questions, thanks!