GPON routers are usually provided for free by the ISP. To reduce the cost, while maintaining complete control of the device, they usually request the OEM to make a customized device with a strange model number. As long as the number of devices is large enough to worth their effort, the OEM will do.
We, as the end users, are given customized boxes which are not available for retail sale. These devices are also available for a limited time, until the ISP finds a better deal with another OEM.
If no one is interested in porting OpenWrt to such device, then it is simply not available. Even Chinese crypto mining routers have OpenWrt -- some have official support, some only have unofficial ports, or variations (X-Wrt).
Your best option is to use your ISP box as a optical to Ethernet bridge.