Diffuser sheet placement?

This set is what's called edge lit, it has 2 led strips on the bottom of the panel, then a defusing screen spreads the light, you have some shorted LED diodes making the darker spots, new lamps should fix the TV as long as the diffuser screen is ok.

If the back light is flickering, you can test and replace the LED in the back light from these links: https://how-to-repair.org/repair/lg-tv-repair-manual-pdf/lg-led-tv-no-backlights-32ln5300-32ln5310-32ln5700-voltage-test-troub/2pyuXwNkBMA.html and https://www.youtube.com/watch?v=_poWTEihIJU. You can download service manual of LG 60LB5900 on this link for reference: LG 60LB5900UV Service Manual free download schematics datasheets eeprom... Fix Led Backlights for TV or Monitor Lg Led Tv No Backlights 32ln5300 32ln5310 32ln5700 Voltage Test Troub Lg...

If you wish to get some details; check the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/
Disassemble & reassemble procedure to many popular brand TVs and other devices are given in detail with illustrations.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electro-medical.blogspot.com/
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

It sounds like you didn't get the diffuser sheets back on in the correct position, so it is not spreading the light correctly. Please keep in mind how fragile these are and the importance of not letting any dust in between the sheets. I wish you well on this endeaver.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electro-medical.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

The most likely problem is the Backlight Inverter. It supplies the power to the LED array which illuminates the screen. If you are brave and also technically able, it can be replaced at home. Here is a video to help. LCD TV Repair Tutorial How to Replace the Backlight Inverter Board in LG...

If audio is OK, the backlight inverter is the part should suspected. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement.

Good Luck.

Most common failures in these LCD monitors are bad capacitors (bulging top/seal or leaking) in the power supply C242 470uF 10V, C241 & C242 1000uF 16V, C261 & C319 680uF 35V failed as indicated in the attached picture, replace them at once.

If the power supply board is working then it is back light problem, first do a flash light test. If you can see the image in the flash light test, follow the steps how to test back light voltages: https://how-to-repair.org/repair/lg-tv-repair-manual-pdf/lg-led-tv-no-backlights-32ln5300-32ln5310-32ln5700-voltage-test-troub/2pyuXwNkBMA.html and follow the steps how to repair back light LEDs: https://www.youtube.com/watch?v=_poWTEihIJU Fix Led Backlights for TV or Monitor Lg Led Tv No Backlights 32ln5300 32ln5310 32ln5700 Voltage Test Troub Lg... Fix Led Backlights for TV or Monitor