According to the FAQ at ASUS https://www.asus.com/ch-en..., the RT-AX89X does not support PON-SFP modules (GPON, XGS-PON). Therefore, this undertaking should not be possible.

I recommend that you check with Salt to see if this is possible, and if so, you can also find instructions there.
Please check with the provider first for such requests instead of asking a community if it depends on your own provider. You can certainly find out such via a support/contact request to the provider instead of asking a community. Moreover, such instructions are only available from the respective provider.
Thank you for your personal enquiry.