Not really clear:
1. What is your old modem and why do you need a new one?
2. What kind of Internet connection do you have (dial-up, cable, ADSL, satellite, 3G/4G)?
3. Why you need both coax in and rj11 in on your new modem?
4. What exactly is "this rj11 connector"?
Not to forget: a modem should be compatible with your provider and preferably they should have instructions on what settings to use to ensure a smooth connection. Before buying anything contact their customer support and ask them if your choice will work.
The modem on newegg you link to goes between the rj11-port in your wall and a USB-port in your PC, using the dial-up technology that I used 30 years for a 300 bps connection, but that was increased to 56 kbps 10 years later.
This was how it sounded: https://freesound.org/people/deleted_user_5789135/downloaded_sounds/ ?
So we have a landline phone that uses rj11 and im currently looking for a new modem
So i need coax in, rj11 in, and Ethernet out.
However i noticed modem selection with an rj11 port are slim and fairly pricey. my question is, is there any way to convert this rj11 connector to an rj45 connector and plug it into something like a switch, or the router itself.
i also found that most of the modems have a usb port on the back as well, could plugging something like this...
https://www.newegg.com/startech-usb56kemh2-dial-up-modem/p/1B4-0087-001H3?Description=rj11 to usb&cm_re=rj11_to_usb-_-1B4-0087-001H3-_-Product
into that usb work as well?