I just had the same problem. I tried from the website and from the T-Mobile app. I'd get confirmation but the change would never go through. I finally called My Team of Experts but because it was past the office hours for My Team of Experts I was transferred to the general customer care team. The rep assisting me had no clue what I was talking about. He told me you could only have one line on your account with full access permissions. I told him he needed to get me the email address for my team of experts so could email them my permission requirements for all 4 of the lines on my account. He told me they had no email address but he could send me a text with a link to schedule a call with them for tomorrow. All lies, I've emailed them before just forgot the address and I never got a text with a link. I ended up going to the chat feature feature in my TMobile app and was answered by My Team of Experts. They changed the permissions on my one line to full access no problem. My advice is to get ahold of them or request transfer to the Tech department. These guys get things done.
The answer to my question marked "Correct" does not answer my question. Both arieswitch and I used the web interface to try to change permissions. Whether it can be done with the 'phone app is not relevant.