Sun Soul Monk gets a special ranged attack using the Martial Arts die for damage. With unarmed attacks and monk weapons it is required that the monk be unarmored and not using a shield in order to use the Martial Arts die. The description of Radiant Sun Bolt does not describe it as a monk weapon or as an unarmed attack. Instead it is a special ranged attack that uses the Martial Arts die for damage.

Does that mean that a monk/fighter in armor and with a shield can use the Radiant Sun Bolt to attack and that they get the Martial Arts die for damage?

The Radiant Sun Bolt feature is not tied to the Martial Arts feature or its armour restrictions, so go ahead and roll that Martial Arts die.

Dang, that's a wild combo I had never even considered. I guess it's balanced out by the fact that a Sun Soul Monk's damage is based on their Monk level and not their overall level, so you can't just do a 3-level dip and get a reliable unlimited ranged radiant attack. I guess it's more useful for taking a dip in another class to get armor proficiency, then going mostly monk. You lose like... 3 levels worth of class features as a result though. I feel like you need to have a very specific build in mind to really take advantage of this.

Seems legal though I had thought not before rereading the feature. I don’t know how powerful it will be since it uses your martial arts die. A one level dip in fighter for armor might be ok as long as you continue as monk for the Ki and bigger damage die. 

But you loose out on many other monk features. If you wanted to be more fighter, getting access to a cantrip would scale better, I would think. 

The difference is that the monk/fighter can use Radiant Sun Bolt on every Attack Action and, for 1 Ki, also on the Bonus Action. A monk/fighter with 3 attacks gets 4 Radiant Sun Bolts.

Can't do that with a cantrip. Cantrips are Cast a Spell Actions, so you can only ever get 1 (well, usually anyway). The closest equivalent cantrip is Eldritch Blast. It does scale with character level, not class level, but IIRC the most you'll cast are 3 (all separate die rolls for 1 cantrip). Might be more damage depending on a couple of other factors.

Dang it, now I've got to explore that possibility.

So, you're right. Eldritch Blast is a d10 damage and you'll eventually get 4 of them (from the single casting of the cantrip). That's an equal number and probably more damage per. Take the Agonizing Blast for the CHA to damage modifier and it beats the monk/fighter combo.

The only way to get three attacks is 11th level fighter which means you max out at 9th level monk. So level 20 you can attack 4 times (3 attack action, 1 bonus action) at 1d6+DEX. Avg 8.5 per attack 34 overall and 1 Ki point. Assuming 20 DEX at this point 

A fire bolt at 20th level, no matter the class combo, is 4d10 or 22 avg damage. So, yeah, not as good. 

A 20th level fighter sword and board is doing 1d8+STR 9.5 per attack avg. or 38 points of damage.

A 20th level monk is doing 1d10 damage unarmed and with the same Ki point expenditure, 4 attacks per round with flurry of blows. So 1d10+DEX or 10.5 avg each and  42 avg overall and they have full use of all monk features that the monk in armor cannot. 

Depends on which direction you want to go. I guess it’s not a bad option if you don’t mind losing monk features.