Unfortunately your choices are basically iframes or javascript aha, personally I use javascript and it isn't too difficult really. If you'd like help just lmk!
I used this tutorial for the Javascript method: https://ikewise.neocities.org/includes.htm The only issue is that some people have js disabled on their browser, and if they do they won't be able to see the nav. You'd probably want to remind people to enable js for your website
If you can't do it on the server, grab a static site generator and use your PC. Jekyll, Hugo, and 11ty seem to be the most popular. I use and recommend htp: http://htp.sourceforge.net/
Thank you!