Why can't it see national cycle routes?

Routing quality depends on your Routing Mode, you've set e.g. Road Cycling, Mixed Surface Cycling etc. and setting "Avoidance Setup" correctly. I've made best results by only selecting "Unpaved Roads" using my Roadbike. If you like unpaved roads, then uncheck this.

I've never seen a difference in routing calculation if "Popularity Routing" is on or off. I've switched it off.

Suggestion... try maybe different Routing Modes ==> calculate "Round trip" with ~20-30km from your home, so you can check quality of proposed routes

Hope this helps

Guido

Ive set mine to avoid dirt roads/gravel roads and to avoid ascents.

What does the unit do?

Send me over exmoor (one of, if not the highest point in all of south england) and down wheel bending (for a fully loaded touring bike) dirt paths that dont even have gravel. Just dirt. I was lucky it was not raining or that would have been mud. I have lost faith in my unit. Getting rid of it.

I'm also excited to have a new Garmin. I was also seeing some seeming strange routing choices so I did some experiments with various options set to try to figure out how it chooses a route.

I found switching routing mode to "Mixed surface" has the biggest effect:

"Road Cycling" mode does make the route tend to roads but cycle paths are allowed if they fit sensibly within a route. The smallest roads (OCM "unclassified") seem to be avoided. This is a shame as these can be nice routes but I'm guessing Garmin made that choice in case they are unpleasant mud tracks etc. Roads marked as residential also tend to be avoided which also is a shame as these could sit naturally on a route but the Garmin is more likely to make large diversions to avoid.

Switching routing mode to "Mixed Surface" generally makes the route use smaller roads, and there is a very strong pull to cycle paths, in particular the national cycle routes. This can sometimes make the route look bizarre as these routes often wiggle their way through town centres linking up town to town and areas within towns. (You can see where these routes are by looking at openstreetmaps.org and selecting the cycle map layer). The "Unclassified" roads not used in road cycling seem to be preferred in mixed surface mode (sometimes at the expense of slightly bigger but also quiet roads).

Switching on popularity routing has quite an effect on the routes I tried. It would generally pull the route towards larger roads (and can cancel the effect of the "Mixed surface" bias towards cycle paths). I'm guessing most cyclists regularly using Garmin devices in my area mainly use main roads.

For me the avoidance settings didn't make any difference. I set toll roads, unpaved and ferries to always avoid and experimented with avoiding (or not) major highways and narrow trails. This had zero effect on my routes.

In summary "Road" Cycling uses mainly larger roads and tends to downplay cycle paths and national cycle routes. "Mixed surface" will emphasise smaller roads, cycle paths and national cycle routes even if it makes the route a lot longer.. Setting popularity routing will make the route tend towards faster roads (at least in my area). 

Hope this is helpful. I'm using Firmware 5.2 and map version 2020.10, and am in the UK.

I always recommend using BaseCamp to aid in understanding route calculations and what impact the different avoidances have.