Scaling Unarmed Attack Homebrew Question

Hi all, first time really getting under the hood of a homebrew subclass here on D&D Beyond. Having some issues with my class feature, hoping to get some feedback.

What the feature does:
Increases unarmed attack damage to 1d8+STR at level 3. Damage increases to 1d10+STR at level 10 and 1d20+STR at level 14. Unarmed strikes count as magical for overcoming resist/immunity at level 6.

What I've tried:
Setting a Modifier (level 3 required, Damage->Unarmed Attacks->1d8->STR score
Result: Changes my unarmed attack damage from 5 (1+STR) to 9 (5+STR)

Based on reading other threads I could find, it seems that this should have worked, but didn't, and I'm not certain why. I can create a custom action and have it scale perfectly with replacements and such, but that doesn't really fit the initial idea of improving unarmed strikes in the literal sense.

Any tips would be welcomed, and if I can't figure it out I'll just have to resign myself to using the custom action.

Thanks, and hope you have a good day.

It wont work that way - the HB engine is a mess. Here is how to do it as easily as possible 

Create a custom cantrip - use shocking grasp as the base (Its a d8) - reflavor the description however you want - change the damage to bludg. Take an extra feat magic initiate whatever - take the new cantrip - manually adjust the to hit and damage modifires (after the cantrip is on your sheet) - and presto - besides having it scale at the same levels as a regular cantrip is better than random levels.