I am attempting to create an ankylosaurus that deals radiant damage along with bludgeoning, but I dont know how much to make it. I have tried looking at other creatures that deal it but those that show up deal it from the weapons they carry not themselves...ideas or places to look?

 In chapter 9 of the DMG, there are guidelines for this sort of thing. (https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#CreatingaMonster) The first suggestion is to find something similar and just adapt it to your needs. (You’ve already naturally started with that idea.) In that same section is a table to help calculate CRs. You figure out the Defensive CR and the Offensive CR from the table and then Average them. (For reference, the standard Anky has a DCR of 1 and an OCR of 4 for an average of 2.5, rounded up to 3.) So now the only real question is: How dangerous do you want it to be?

If you want it to be the same CR as the standard ankylosis, then just split the 4d6+4 bludgeoning into 2d6+4 bludgeoning and 2d6 radiant. (Or you could keep all of that bludgeoning and just add 1d6 radiant, that would keep it the same CR.)

If you want your Anky to be strong enough to come in at CR4 based on nothing but the additional damage,  then that will take between 3d6-5d6 additional damage per attack on top of what they already do. (That’s because it would have to get the total average damage up to the  27-32 range for the calculations to work out because that table kinda sucks eggs.) Conversely, you could keep it at 2d6-3d6 additional damage and raise the AC to 17 and that would also bring it in at CR 4.

If you want your custom Anky to weigh in at anything above CR5, then that would require additional HP, a higher Attack modifier, and maybe some kind of additional special ability. (Or it would require enough damage per attack to one-shot kill an 11th-level Wiz/Sorce, which you probably don’t want.)

I hope that helps.

That helps a good deal thanks! I had started thinking 1d or 2d but thought it would b under powered ( my original thought took me to pike and her mace of disruption.)

Am I correct in assuming that radiant resistance would effect the CR lvl as well? It is my assumption that a creature that deals a type of damage would be resistant to it. 

What it boils down to is that these anky's have evolved next to a tear spilling out radiant energy. They have also been harnessed by a local civilization as military mounts ( I'm assuming there would be a difference between the AC of "standard" version and one outfitted for military use. )

Quote from Quev >>

1. That helps a good deal thanks! I had started thinking 1d or 2d but thought it would b under powered ( my original thought took me to pike and her mace of disruption.)
2. Am I correct in assuming that radiant resistance would effect the CR lvl as well? It is my assumption that a creature that deals a type of damage would be resistant to it. 
3. What it boils down to is that these anky's have evolved next to a tear spilling out radiant energy. They have also been harnessed by a local civilization as military mounts ( I'm assuming there would be a difference between the AC of "standard" version and one outfitted for military use. )

1.  Mace of Disruption is a Rare Magic Item. Magic Items and Monsters are completely dissimilar in almost every way they can be and still function in the same game system. That’s why the DMG has entirely different entries For the various homebrews.
2. Being able to deal a damage of a certain type doesn’t automatically confer resistance to that damage type.
   Damage Resistance is not listed as affecting CR. 
3. There are rules for Barding, and they are the same as regular Armor. That means those Ankys wild need Plate for any armor to be better than their natural AC calculation. (Like how a Tortle gains no benefit from Unarmored Defense until that hits 18+.

You should really look through chapter 9 of the DMG, it will likely answer most of your questions.