Is this your pc?
https://www.lenovo.com/au/en/deskto...series-sff/ThinkCentre-M720-SFF/p/11TC1MDM72S
I think whats happened is the lenovo's bios was setup to run both Intel and Nvidia graphics simultaneously as in multi monitor's using independent video ports unique to both graphics. Usually motherboards disable igpu if a graphics card is detected which leads me to think lenovo sets the bios manually before shipping it out not expecting customers to be removing cmos battery. That's my guess, igpu remains disabled even though you've reset cmos because the GT730 is in there still.
The hard part for me trying to help is understanding where the GT730 is in the case. Does this picture look like your case? At the rear where there are four covered slots above the power plug, has one of those covers been replaced by GT730? If so, try one of those ports if you have Displayport.
The specs say GT730 has two Displayport video ports and igpu has a two x Displayport + vga video ports. Igpu being disabled, both Displayport port and vga belonging to igpu would be disabled. GT730's Displayport ports should still be active.
Which video connection are you using with your monitor?
Vga:
Displayport:
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If you are using Vga only monitor, that makes it hard. To get igpu back online, remove the GT730 so the bios re-enables it by default. The tricky part is if you put back the GT730, the igpu will most likely be disabled again if Vga is what you're using. Need a Displayport cable so you can reconfigure the bios and enable igpu using one of the GT730's Displayport's.