How do you automate putting fuel in the heating towers (the ones that spawn on the planet). I "solved" the planet, launched the hydrogen bomb and left, but everything froze over when I was gone because I wasn't around to refuel the heating towers.

I tried having a belt with solid fuel and a chest with solid fuel with an inserter next to a heating tower but that did nothing.

If inserters cannot insert solid fuel into the towers, you have another mod on your save that is breaking Cerys.

I went back to Cerys to further investigate issue. The problem was I expected the heating towers to act like the regular Heating Tower, with inserters filling it to the brim if possible. Instead, they seem to act like boilers, where fuel is only inserted when it is almost out (<5 solid fuel).
So the chest next to the tower works fine, just not in the way I expected.