You’ll have to do a bit more than just removing the 301 redirect since the entry doesn’t have any PHP support included.

Instead, copy the contents of the SSL virtualhost in the other file in /etc/nginx/sites-enabled/ replacing the servername and return 301 lines here. Then delete the lines covering your certificate files. Once done, restart nginx with service nginx restart

Ex. You’ll want to remove the lines that look like this:

        ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;

as well as making sure you don’t leave anything referring to port 443. Your default-ssl.conf file will not be changed in any way, we’ll just use it as a source for the configuration directives we need.

I think your problem may be- because you are using
add_header Strict-Transport-Security max-age=15768000;
This is actually telling the browser to choose https over http for the domain name requested. No matter if your configuration files have both encrypted and non encrypted traffic, this directive tells the browser to serve https over http
And you can use: return 301 $scheme://$servername$requesturi;
instead of: return 301 https://$servername$requesturi;
See here
Also are you creating seperate nginx configuration files for each domains like /etc/nginx/sites-available/maindomain for main domain
/etc/nginx/sites-available/anotherdomain for another domain?
and symbolic links to sites-enabled? I’m sure you’ve done it already but still asking to make sure.
This tutorial is more superior than some others in creating ssl websites with letencrypt.

Thank you so much for responding. My two files look like this

server {
listen 443 ssl;
    server_name maindomain.com *.maindomain.com;
    ssl_certificate /etc/letsencrypt/live/maindomain.com-0001/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/maindomain.com-0001/privkey.pem;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_prefer_server_ciphers on;
    ssl_dhparam /etc/ssl/certs/dhparam.pem;
    ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-S$
     ssl_session_timeout 1d;
     ssl_session_cache shared:SSL:50m;
     ssl_stapling on;
     ssl_stapling_verify on;
     add_header Strict-Transport-Security max-age=15768000;
root /var/www/html/wordpress;
    index index.php index.html index.htm;
    location / {
        try_files $uri $uri/ /index.php?$args ;
    }
    location ~ /favicon.ico {
        access_log off;
        log_not_found off;
    }
        location ~ /.well-known {
                allow all;
        }

And The second file is:

server {
listen 80;
servername maindomain.com www.maindomain.com;
return 301 https://$servername$request_uri;
}

Are you saying that I should remove all ssl related lines in my port 443? Or that I shouldn’t have anything related to port 443. Instead let the server treat everything as HTTP?

Should there be just one file that looks like this:

server {
    listen   80;
    listen   [::]:80;

    server_name maindomain.com *.maindomain.com;

    root /var/www/html/wordpress;
    index index.php index.html index.htm;
    location / {
        try_files $uri $uri/ /index.php?$args ;
    }
    location ~ /favicon.ico {
        access_log off;
        log_not_found off;
    }
        location ~ /.well-known {
                allow all;
    }
}