Before I spend a bunch of time on this, I wanted to verify what I’ve read in older posts is still accurate and there is no work around. It is impossible to sync 2 guides at the same time (one for cable and one for OTA) on a single Plex server? The recommended solution is to setup a second server to record from the second box?
I found that if you use the Cable guide, you can assign the corresponding channels from cable to the tuner channels. In the latest firmware, it works like a charm. We now have 3 prime tuners and 2 extend tuners using the cable channel guide to pull content. The downside is that a couple channels were not matched from tuner. This was mostly out of market (60 miles away) and less necessary as it is often duplicative outside local news.
Alternately, you can set up your guide for a market that you know has all of your channels on cable, then hand enter each guide channel to each channel in your DVR & Live TV settings. Hard work and prone to breaks once a year or so, but you solve your problem.