Unable to activate iMessages with my very old iPhone 4S after transferring my phone number.

Hello.

Yesterday, I transferred my old Verizon Wireless cell phone number from my old iPhone 4S (iOS v9.3.6; got it in early 2015) to my new used iPhone 6+. Obviously, I can't use its SMS but I can still use Apple's iMessages. However, iPhone 4S' iMessage Activation keep says "An error occurred during activation. Try again." even if I use my Apple ID account for its iMessages. "Send & Receive" is using my Apple ID e-mail address, and not my old Verizon Wireless former cell phone number. I already tried signing out and rebooting. None of them help. Also, I'm using its US region server. Once, I did get its iMessage Activation error saying "Unable to contact the iMessage server. Try again."

How do I fix this so I can still iMessage on my iPhone 4S? Thank you for reading and hopefully answering soon. :)