Resolve SignUp Functionality Issues

Fix sign up.

Context:
Optin builder so users subscribe to a paid plan to unlock content.
They have two paid options: purchase annual subscription ; buy credits and use those credits to unlock one article
When you scroll back in the article you see two bottons :
Botton A: Upgrade now -> redirect them to the plan page -> this workflow is ok (choose plan, fill subscription form, OTP, pay)
Botton B: buy article. There are two cases ; case 1: user logs in and if he click on buy article and has credits , it means he will use his credits to unlock that article. case 2: user doesn't have an account and wants to sign up -> he fills the form -> the sign up doesn't work .. This is what we need to fix

Problems :
- Maybe the sign up of botton B doesn't work because the sign up form doesn't know which plan he selected as the user selected no plan .. or it could be due to another issue.
- Optin builder is already very customized and I don't want it to be more customized (I have as project to install child theme .. to eleminate the issue of code loss when I update pluggins) -> here you need to tell me if I install child theme after this milestone, are we going to loose the work already done?

How to fix:
My proposition is if user clicks on botton B
Case 1, he logs in -> all ok
Case 2, he clicks on sign up -> just redirect to the plan page -> I hope this will not lead to more customization, want to keep things simple.

Normally here, I'm clear, if not clear, ask me questions.

Deadline: 1 day should be enough.
Job done = sign up works perfectly, no bug, no code deleted, no issue, all is working.
Expected at the end of the task: fill a short word document to explain to other Devs what you did, where you placed the code, and any information you find useful for another dev working on the project.