This may take some time but I would disassemble what you have repaired and check the antenna ports to make sure you did not accidentally damage the port on the logic board. I have done it and it still works but not every one is lucky . iFixit suggests cleaning the grounding locations as finger oils can prevent the grounding and thus you would have this issue. If you are doing this for a water damage phone make sure there are no signs of corrosion on the Logic board or the case where.
Here is my list of what I do. Try cleaning the charging port with Isopropyl. Make sure the pins in the charger are not stuck Try a new battery and charger. Try a new DC in board. If none of those work then there is something going on on the logic board and at that point you need skills in board repair.
Why buy them online. What I do is ask friends or family members. If that doesn't work stake out your local e recycling place. Often times people take repairable devices there just to clear space. If push comes to shove visit a repair shop/ good will/ salvation army and let them know what you are doing and if you can have their junk phones or other devices. You probably wont get anything usable but you may get some good practice devices.
I would say follow the basics like resetting the SMC. https://support.apple.com/en-us/HT201295 On older models there used to be LEDS on the Main Board the showed if it was getting power and such but you probably want to give it a visual inspection to make sure there are no obvious faults like burnt spots. Unfortunately you will probably need a Multimeter, the Schematics for the iMac and knowledge of how to read Schematics. Worst case scenario is the that the CPU or Graphics card. If its the PSU then that is a relatively easy swap but if there is a component on the Main Logic board that died then you would more than likely need good Microsoldering skills as component level repair is not so easy.
If you have another mac you can follow these steps to make a bootable USB with any of the OS installers in the purchased tab in the App Store. ( preferably the same OS you are on or newer) https://support.apple.com/en-us/HT201372 If you can get to recovery by holding down Command + R or Command + Option + R or use the USB you just created. I would first go to Disk Utility and find the Main Hard Drive which is named Macintosh HD by default. Then click First Aid, Verify or Repair disk. If the drive is OK then you just need to go back to the main screen and reinstall OS X to fix it without losing data. If you don't care about data you made after your Time Machine Back up then restore from Time Machine. If the drive fails I recommend getting a cheap SSD and use the USB or Time Machine drive to format it to Mac OS Journaled then restore your Time Machine to it. ***NOTE***Did you know you can boot directly from your Time Machine? If all else fails hold down the option key when you start the Mac while your back...