It has been taking the IRS a very long time to process mailed returns. And since your return is a 2018 return, nothing is ever going to show up on the IRS refund site. Only information for 2020 returns is shown there. Did you use a mailing service to track it so that you would know they received it when you mailed it last year?
Only the IRS can provide information to you:
To call the IRS:IRS: 800-829-1040 hours 7 AM - 7 PM local time Mon-Fri
Listen to each menu before making the selection.
First choose your language. Press 1 for English.
Then do NOT choose the first choice re: "Refund", or it will send you to an automated phone line.
Instead, press 2 for "personal income tax".
Then press 1 for "form, tax history, or payment".
Then press 3 "for all other questions."
Then press 2 "for all other questions." It should then transfer you to an agent.
Did you check your account on the IRS site ... did it ever get entered into the system and processed ?