It was my understanding that the weapons level every 100 levels from the point you buy it. However some people are saying you need to buy a new weapon every 100 levels. Can someone clarify which it is?
Does the weapon level with you? or is it just stuck at the spot when you buy it?

from my understanding it's stats are set based on your level at the time you EQUIP it. It's stats go up every 100 levels and max out at 500+ levels. So basically don't bother till you get to level 500.

I bought these Radiant Surge Cannons when i was level 40 and now that im 107 attack remains 40.. and now im confused!

The radiant weapon goes off when you buy it. It does not go up in value as you rank up. If you buy one at rank 1 the attack will give you 40 even if you keep it on till rank 300. In order to get it to give you more attack you would have to scrap those and buy a new one once you get to rank 100 +. If you read what it says when you buy them you will get it. It says this "Installs a Radiant Surge Cannon on your ship (limit 2 per ship).
Scales in attack and size based on your rank when first installed. This scaling occurs every 100 ranks."

You will notice it says when the weapon is FIRST installed. Stating that it will not change even if you pass the scaling point. I hope this makes since. lol

My Radiant Weapon has +200 attack points and I purchased when my rank reached 500. I would say they aren't worth it and you would get a lot more value out of a quasi expander to make a great planet larger and increase your resource production

In M101x's case you can use past tense : ) He royally screwed himself already.

I agree totally with this.

The stats are based on the level that you equip them at. They do not increase with your level. So either buy two new Cannons every 100 levels, or wait until you are level 500 as Kaos suggested.

I've updated the wiki to make this clearer.