Willaing want to use the MR200 as a modem, that is to have the MR200 establish only the LTE "physical" layer. WAN interface Sorry for my mistake. You want to use your MR200 like a modem and second...

Firstly and more important question, Do you have public IP address from your ISP?

I'm not sure for 100% but it seems to me that telnet service is blocked on this router.

W8961 doesn't support WAN connection. You can setup W8961 only like a AP (access point). Very important in this case is Fiber modem and his configuration (modem or router).

This is too old router for WPS.

I think that your upload is too small for the division of your internet link. All time will be something wrong.