To get at the part the TV must be laid on the screen surface (table with soft towels and smooth surface is advised) and about 20 tiny screws must be removed in order to remove the bottom part of the back cover. Take a close look at the screws as you remove them, some are self-tapping, some are machine screws. And there are a few that are shorter and must go back in the holes they came out of.
Once all the screws are out, find something to pry under the rear cover and gently flip it so the bottom edges of the cover and the TV stay together. There are cables connecting to the loudspeakers which are attached to the rear cover. If you have it positioned right, the back can stay where it is and no need to remove the cables.
You'll locate the WiFi board in a plastic base, mounted to the main chassis near where the left side speaker would be. There is a clip holding the WiFi board in the plastic base, and a cable connector (pull straight out) and an antenna connector (use a thumbnail under the lip on it after removing the board.)
Remove the WiFi board.
This is a case where reassembly is the reverse of disassembly. Nothing terribly complex about it - take your time. After installing the board, I did plug the set in with the back open (don't touch anything inside while power is connected) and made sure I could sync the WiFi remote to the TV. Since it linked almost immediately - I was satisfied that the WiFi connection to my network would work, and I was correct.
It's all working fine now. In a photo of one of these boards used on eBay, there was some white sealant put over the antenna connection to the WiFi board. I suspect this connection may become loose with time due to it's proximity to vibrations from the loudspeaker.
Chances are good that the set can be fixed by simply reseating this antenna connection, but since the replacement board was so cheap, I wanted to have it in hand and just replace it so I was sure it would be working.
FWIW - the company providing this part also has a kit for ALL the circuit boards in the TV - for only $69. I bought one to just put on the shelf in case any other issues ever arise with the set. It's TCL 55R617 55R615 55R613 Complete TV Repair Parts Kit Version 1 sj-KIT-55R617-K1 1
ShopJimmy has outstanding prices and the parts arrived within 3 days. I recommend them. The parts all appeared brand new, in anti-static bags, and wrapped in paper and then boxed.
The symptoms of this failure are: difficult or no connection to "enhanced" Roku (voice activated, WiFi) remote and difficult or no connection to your WiFi network.