In reply to a post by jchamier:

I don’t think you’ll find anything. Not really a demand: except for those products with network operator firmware that I don’t think any support a fixed line telephone port.

Indeed it's the firmware (and network support) that are lacking.

For example the Huawei CPE Pro 2 (PN: H122-373) - a very popular and capable 5G router - has an analogue RJ11 port present but none of the known firmware versions it ships with support it.