How many UART lines should I connect between module and host
CASE 1
Send and receive data and SMS.
Use TXD and RXD

CASE 2
Send and receive data and SMS + Microcontroller goes to sleep and needs to be awaken up by the LTE module.
Use TXD, RXD, CTS
The CTS signal goes down when and SMS arrived or when a phone call is arriving. It's called network PAGING.

CASE 3
Send and receive data and SMS + hardware flow control during LTE/3G/GPRS/2G data transmissions.
Use TXD, RXD, RTS, CTS

CASE 4
Send and receive data and SMS + simultaneous phone calls.
Use TXD, RXD of the first UART (data + SMS)
Use TXD, RXD of the second UART (phone call answer and hang-up)
The second UART is needed if you want to send or receive SMS messages during a phone call.

CASE 5
Send and receive data and SMS + simultaneous phone calls.
Use TXD, RXD of the first UART (data + SMS + phone call answer and hang-up)
Write a piece of C code that implements the client side of the 3GPP mux protocol.
This is tough if you are a newbie.
The 3GPP mux protocol will allow you, using just one UART interface, to send and receive SMS messages while a phone call is in progress, which is usually not possible with just one UART.

DTR and DSR are supported by the LTE module.
They are useful only if you want to connect your LTE module to a PC that runs a very old telephony software.

I've never used the RING signal because I always waited for the +CLIP unsolicited indication, arriving from the UART, that contained the actual caller's ID.

I've never used the DCD signal because I always waited for the +CONNECT unsolicited indication,  arriving from the UART, that contained the connection speed expressed in bits per second.