(PHP Way) This plugin insert to html missing image alt tags/attributes based on each attachment title or filename.

wodey

Auto WebP & Alt Optimizer

Automatically convert uploaded images to WebP format using native GD library for maximum compatibility, and auto-fill image Alt text for better SEO.

Kamer DINC

Compatible with the most popular plugins. Please report if you faced any problem with any plugin. 
Inserts automatically alt tag/attributes to HTML if alt attributes are empty or somehow missing. 
Works only in single page.
Note: This plugin is written only in PHP. So you can except a better SEO visibility and better crawl statistics.
Credits

Alternatives to WP Auto Alt Image Tag

Top alternatives