LD200RM REMOTE CONTROL DOESNT WORK ON MY APEX LE3212D

How would we know that??

If you wish to buy its original remote control, contact the authorized service centre, or authorized dealer to your device brand. They can either give you a genuine remote control to your device, or can direct you, from where can you purchase a suitable one.
To select a proper universal remote control, you should know about the number of digits to its set-up codes. There are 3 digit, 4 digit, 5 digit, 2 digit and even single digit codes, to some brand devices. After confirming this, check the sites linked here. Both the sites have set-up code list and set-up procedure to many popular brand remote controls. Check which brand remote control has set-up code/s to your device brand, and buy that one according to availability. Brand name of the remote control isn't a matter, but it should have set-up codes in its code list, to the device brand you have.
Please note: Every new universal remote control will have set-up code list, and set-up procedure along with its pack. Never accept a remote control, that hasn't both of this. For more details about the set-up codes, visit the sites.
http://electrohelponline.blogspot.com/
http://remotesearch.blogspot.com/
Universal remote controls are available at most of the electronic spare part selling shops. Visit some on-line sites too, which deals with electronic spare parts.
If you wish to get further advice over phone; visit https://www.6ya.com/

Fault to the main power supply regulator board. Check and replace damaged component/s at its main power regulator [SMPS] section circuit. Contact any service technician. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electronicshelponline.blogspot.com/
It will be best to replace the power supply regulator board as card basis rather than making "Surgery" to it; which usually will make loss for both money and time. Even though you can make it right, it will cost around an amount equal to the cost of a new board; including the cost of (spare parts + service charge) More to say, most of the spare parts to it will be difficult to get from open electronic spare part market. This is true to most of the flat panel TV power supply boards; irrespective of its brand and screen size.

You should enter its service mode to do this.
If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/
Service mode entry, adjust and exit procedure to many popular brand TVs are given. Never forget to exit the service mode, after all adjustments have been done.

I know this is old but I figured I can post an answer for anyone else looking for a solution. I don't have a remote for my Apex tv and wanted to switch it from TV input to Hdmi 1 to use it as a monitor. You have to press INPUT button to get the menu up. Then press CHANNEL+ or CHANNEL- to change to the input that you want. Then press and HOLD the VOLUME+ button to enter it. That worked for my Apex TV.

You can do a code search method. If you wish to know, how it should be done, check the sites linked here. One among the posts have details. http://remotesearch.blogspot.com/
http://electrohelponline.blogspot.com/

Check and replace damaged component/s in its main power regulator board. It will be best to replace this board as card basis. If you are not familiar with electronics, do not attempt to repair. Whether you suffer fatal electrical shock! Instead, contact the nearest service center.

it should be 3 or 4

RCR450 have no setup codes to Apex TV. There is no RCA remote control models have set up codes for Apex brand.

There is no setup codes for Apex or RCA STBs with this GE remote control.