Good Day, Approximately * months ago, our Arris modem*Access point was traded out for...

Good Day, Approximately * months ago, our Arris modem*Access point was traded out for the Arris Tm-**** and seperate Wireless RAC*VIS Sagecom. Since then we have had timing out issues with SMTP email being served by our domain. This is happening on multiple computers using different Email clients. One has the latest version of Thunderbird, the Other has **** Outlook. When sending mail the SMTP servers time out before connecting. We have Tried Imap and secure server settings, along with standard POP**SMTP settings changing the appropriate ports. Upon sending a message the system waits approximately * minute, and then either times out, or finally sends. Upon time out, if you resend the message, it goes through the same procedure, sometimes finally sending and other times, time out failing. Sending mail using the same clients (Thunderbird and Outlook ****), using gmail as the smtp network works fine. NOTE: This is NOT Brighthouse or Spectrum email. This is email accounts hosted on my domain, which worked fine until the Modem*Router*Accesspoint was changed out. Are their ports being blocked that might cause this by Spectrum*Brighthouse? Why would changing a modem*router cause this?*Can I get another Modem*Access point to see if it will correct my problem? How do you suggest I correct this issue that has been going on far to long?