Images and css files not loading on live server

jreklund thanks for your guidance with this problem.
I have now found a resolution albeit not ideal.
On my live web server account I am running with php 5.6 set to the current php version and I was using this line in my .htaccess file located in the project root:
SetHandler application/x-httpd-ea-php73
This was originally used in my wordpress site on the live server to allow it to run with the newer php version and so I thought that it would also work for Codeigniter 4. Howwever this was the cause of me chasing around for 2 days trying to track down the problem!
I then thought that maybe I could get away with making php7.3 the current php version on the web server and using a similar line in each of the php5.6 codeigniter2/3 web apps .htaccess like this:
SetHandler application/x-httpd-ea-php56
to overcome the problem. Unfortunately all that did was shift the problem to all of the php5.6 codeigniter web apps!!
Not sure what it is about using this SetHandler on the web server that causes the issue but obviously it does not provide a 100% php version for some reason.
So it looks like all I can do for now is work locally to learn Codeigniter 4 and then rewrite all of my old codeigniter 2/3 web apps and then endure some downtime while I change the server back to php 7 and attempt to deploy them in a live environment - not what I had hoped to do at all.
If anything I have said here rings any alarm bells or you can think of anyway other way of working with two different php versions on the live server then please share your thoughts.
Thanks in advance.