Trying to change my profile picture back to the default initials have tried and no success

This is your best option afaik...
https://ui-avatars.com/

Here is an example
https://ui-avatars.com/api/?name=Chuck+Strand&background=0D8ABC&color=fff&rounded=true&bold=true&size=300&font-size=0.70

In this 2nd link you can identify several keywords:
name=Chuck+Strand
background=0D8ABC
color=fff
rounded=true
bold=true
size=300
font-size=0.70

you can edit those to be whatever you want. For colors you can find the codes using this HTML color code picker https://html-color-codes.info/
For font-size use anything between 0.01 and 1.00.

There was a much more convenient app but, unfortunately it is down. RingCentral does not offer a way to revert the profile pic. I've searched a lot about this a few months ago :D