I use WordPress with Invision Power Board but I would like to change my home page. I have the installation of WordPress in the main directory / and the installation of Invision Power Board in the subdirectory /invision/. Until now I have left the WordPress installation htaccess intact, which sets the WordPress index.php as the Home page. But now I would like to change the home page and choose as the Home page the Invision Power Board address, which is located in the / invision / subdirectory. I changed the .htaccess to set the home page in the invision subdirectory. This is the modified htaccess:

#Alternate default index page
DirectoryIndex invision/index.php

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^ index php $ – [L]
RewriteCond% {REQUEST_FILENAME}! -F
RewriteCond% {REQUEST_FILENAME}! -D
RewriteRule. /index.php [L]
</ IfModule>

# END WordPress

But I only got the following error:

Forbidden
You do not have permission to access this document.
Web Server at pets-99.com

I get the same error if I use the code :

#Alternate default index page
DirectoryIndex invision

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^ index php $ – [L]
RewriteCond% {REQUEST_FILENAME}! -F
RewriteCond% {REQUEST_FILENAME}! -D
RewriteRule. /index.php [L]
</ IfModule>

# END WordPress

To avoid the error i restored the original htaccess. That is this:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^ index php $ – [L]
RewriteCond% {REQUEST_FILENAME}! -F
RewriteCond% {REQUEST_FILENAME}! -D
RewriteRule. /index.php [L]
</ IfModule>

The problem is that now I no longer get the mentioned error but as a home page if I use https://www.pets-99.com/
I get the Invision Home Page and I see the Invision Home Page as WordPress Main Page too. While I get the main page of WordPress if I write http:// instead of https:// . If I use http://www.pets-99.com/ the main page of WordPress appears. If instead I use the link https://www.pets-99.com/ I get the Invision Power Board main page. Is it a cache problem? So it’s a temporary mistake because I restored the original .htaccess.
How can I change the home page from WordPress to Invision? Can someone help me ? How should I adjust the .htaccess?
I use Let’s Encrypt SSL/TLS Certificate, with the ultimate version of Plesk, and PHP 7.1.

Is the landing page of IPS set to the Page you need it to land as Home?