Hello,
The SFP port on RUTXR1 can be used with standard SFP transceivers (SFP up to 1 Gbps is supports, SFP+ not). It can be any standard 3rd party SFP adapter, including multimode or single mode, but there might be some SFP adapters on the market that are designed to be compatible with specific devices. The recommendation is to avoid such adapters. Hence, I cannot guarantee that the SFP module that you have linked will work, but if you have it, you can give it a try. The standard GPON SFP Gbic modules may potentially work, though GPON ONT SFP has not been tested, so there is no guarantee.
There were a few SFP modules that were tested and were confirmed to work, such as the ones here and here. These are single mode, but there should not be any issues with multimode.
However, since you are looking to replace ONT/ONU, I would strongly recommend contacting your ISP to see if you can use an SFP directly, and if so, what SFP should you use as these need to match on both sides. The ISP may use the MAC address, Serial number, and many other parameters to identify your ONT. So, the best approach would be to contact your ISP and see if you can use GPON ONT SFP Gbic in your case.
Kind Regards,
Andzej