@Orbital I suggest you read the upgrade notes for RRF 3.5.2 and whatever versions you skipped. From the symptoms you describe I suspect you have a M667 command in config.g that needs to be replaced by a suitable M669 command.
Download that zip file and extract it's contents to the WWW folder on your SD card. Then you should be able to follow along with the rest of the getting connected guide.
If you don't have a WWW folder on the root of the SD card, create it and extract that zip file into it.