Configuring the plugin
Where can I configure the WooCommerce Related Products By Attributes plugin?
Go to WooCommerce -> Settings -> Products. Scroll down to the 'Related Products by Attributes' section to configure the plugin.
Randomizing related products is disabled, but still they seem randomized
Some theme developers randomize related products within their theme code too. Unfortunately, we cannot influence their code. You could check this by previewing your shop with a default theme like TwentyTwenty. If the behaviour doesn't occur there, it's definitely a theme issue. Please check your theme settings, or ask your theme developer to disable randomizing related products at all (since WooCommerce already does this for you).