@Dr_Andy  Hello, 

Thanks for your detailed description, but I'm afraid we do not have the firmware of this model either. 

Archer XR500v is a customized model sold in certain areas or supplied by particular ISPs. (more like a ISP edition )
If you purchased this modem from ISP, it is recommended to contact ISP side and report the case details, so they could help check it further. 
If you purchased this modem from local retailers (not ISP side), please contact our local support and see if they could help. 
https://www.tp-link.com/support/contact-technical-support/#Hotline-Support

Besides,  uploading third party softwares may affect the device warranty & further tech support from tp-link,  if local support could not help, you could  contact retailer and see if they could exchange one. 

Your understanding is much appreciated.