Unable to send email from Thunderbird after Network Solutions changed SMTP server

Hello. I have been unable to send email from one of several POP accounts on Thunderbird (v 115.18.0). I believe this occurred after Network Solutions changed the SMTP server for that email account. I am able to send email from my Gmail IMAP account setup on the same Thunderbird program on my PC. I have tried all the port and authentication settings for SLS/TLS and STARTTLS, changed/updated passwords, etc. I do not get any error messages, just never receive the test email I send to my other accounts. I updated the SMTP server on my iPhone mail app to the new NetSol SMTP and it works on that platform. I just cannot get it to work on Thunderbird on my PC. NetSol support said to delete account and start over which is not possible as it is my business email account. Also, the inbox is too large to back-up to an external drive. Any suggestions? Thanks in advance. John