The traffic between and USG and AR through IPsec VPN tunnel is not passing, even though the IPsec is established.
Check the IPsec proposals and IKE proposals configuration.
Check the output of display IKE sa command and display IPsec proposal to check whether both ends have IKE set up and if they have the same algorithms and parameters configured.
Make sure the encryption algorithms are the same on both ends. Also the authentication algorithms.
Command "ipsec authentication <encryption_algorithm> compatible enable" is not configured, therefore the traffic is interrupted when connecting the USG and AR, becuase they are different devices.
Configure ipsec authentication <encryption_algorithm> compatible enable on both devices and test again using "display ipsec statistics esp".