Using nmcli to set subnet mask and gateway IP

The Red Hat Enterprise Linux 7 Networking Guide (see link) says that the following syntax should be used to assign a subnet to a gateway:

nmcli connection modify eth0 +ipv4.routes “192.168.122.0/24  10.0.0.1”

How do I adjust the above to accommodate the 255.255.255.248 Subnet Mask and the aa.aa.aaa.aa6 Default Gateway given by an internet service provider? Something like:

nmcli connection modify eth0 +ipv4.routes “255.255.255.248  aa.aa.aaa.aa6”

Note that:

1. the subnet is given in two different formats.
   a. The RHEL7 Networking Guide gives the subnet in the format 192.168.122.0/24, while
   b. the internet service provider gives the subnet in the format 255.255.255.248.

2. What is more,
   a. the RHEL7 Networking Guide gives the gateway as a private IP, while
   b. the internet service provider gives the gateway as a public IP.

For reference, a Windows laptop is able to connect to the internet and be recognized as one of the 5 valid IP addresses by giving the above information using the following steps:

1.  Control Panel > Network and Internet > Network and Sharing Center    
2.  Click “Change Adapter Settings”  
3.  Right click on “Ethernet 2” connection and click on “Properties”  
4.  Select “Internet Protocol Version 4 (TCP/IPv4)”  
5.  Then click on “Properties” Button to open the target dialog box:  
    a.  In the default state, the “Obtain IP address automatically” option is checked  
    b.  To claim a specific IP instead, click “Use The Following IP Address” and enter the following information:  
            i.  IP Address:  aa.aa.aaa.aa1  
            ii. Subnet Mask: 255.255.255.248  
            iii.    Default Gateway: aa.aa.aaa.aa6  
            iv. Preferred DNS Server: bb.bb.bb.bb  
            v.  Alternate DNS Server: bb.bb.cc.cc  
            vi. Check the “Validate Settings on Exit” option.  
            vii.    Click OK  
6.  Click on any other open dialog boxes to return computer to normal state