After few months I found a way to install Open WRT software on Buffalo Router WSR-3200AX4S at the below link:
Unfortunately all instructions are in Japanese (can overcome with Translate) but they are working 100% for WSR-3200AX4S, installed and so far no issues found. Thanks to developer @ssyysy2021 from above link.
The solution is suitable also for
- Buffalo [WSR-2533DHP3]
- ELECOM [WRC-G01]
Hope it will help some other people.