To add or configure extra IP ubuntu 20.04, kindly follow the below instructions:

Step 1: Find Netplan Configuration File, Look for Netplan's configuration file using:

SSH: ls /etc/netplan/

Step 2: Open this file with a text editor, for example, note: here: 50-cloud-init.yaml it can be something else this file name you will get from step 1:

SSH: sudo nano /etc/netplan/50-cloud-init.yaml

Step 3: Suppose your Netplan configuration file looks similar to this:

network:
   version: 2
   ethernets:
       ens3:
           accept-ra: false
           addresses:
           - 2001:41d0:401:3000::2adf/56
           dhcp4: true
           match:
               macaddress: fa:16:3e:7c:d6:26
           mtu: 1500
           nameservers:
               addresses:
               - 213.186.33.99
               search: []
           routes:
           -   to: ::/0
               via: 2001:41d0:401:3000::1
           set-name: ens3

Step 4: Now add your extra IP like below, after adding IP press ctrl+x & save plus exit it:

network:
   version: 2
   ethernets:
       ens3:
           accept-ra: false
           addresses:
           - 2001:41d0:401:3000::2adf/56
           - 149.202.53.205/32 #New Extra IP
           dhcp4: true
           match:
               macaddress: fa:16:3e:7c:d6:26
           mtu: 1500
           nameservers:
               addresses:
               - 213.186.33.99
               search: []
           routes:
           -   to: ::/0
               via: 2001:41d0:401:3000::1
           set-name: ens3

Step 5: Apply Changes, After saving the file:

SSH: sudo netplan apply

Step 6: Verify New Configuration, Check that the new IP address has been applied:

SSH: ip a

You are done, your extra IP will ping now.