Platform: CISCO2921 IOS version: 15.3(3)M5

Load the anyconnect package on the flash of the router and configure anyconnect client package (be patient, this may take a while...):

crypto vpn anyconnect flash0:/webvpn/anyconnect-win-4.1.04011-k9.pkg sequence 1
SSLVPN Package SSL-VPN-Client (seq:1): installed successfully …