If you're not allowing Solid Security to write to the .htaccess file, or aren't able to, these rules will need to be manually added. Just copy and paste the rules at the top of the file.
# BEGIN iThemes Security - Do not modify or remove this line
# iThemes Security Config Details: 2
# Ban Hosts - Security > Settings > Banned Users
SetEnvIF REMOTE_ADDR "^54\.37\.1\.123$" DenyAccess
SetEnvIF X-FORWARDED-FOR "^54\.37\.1\.123$" DenyAccess
SetEnvIF X-CLUSTER-CLIENT-IP "^54\.37\.1\.123$" DenyAccess
<IfModule mod_authz_core.c>
<RequireAll>
Require all granted
Require not env DenyAccess
Require not ip 54.37.1.123
</RequireAll>
</IfModule>
<IfModule !mod_authz_core.c>
Order allow,deny
Allow from all
Deny from env=DenyAccess
Deny from 54.37.1.123
</IfModule>
# END iThemes Security - Do not modify or remove this line