Marios,
You need to put the modem/router into bridge mode. This way it will pass your public ip address over to the RV042 and this problem won't occur. Currently right now, you are NATTING twice all traffic coming inbound and can cause certain routing problems and other various problems as well. Once you get RV042 with public ip address your DDNS account will start reflecting the proper address.
Or if you can’t bridge your modem/router, see if there is a place in that device to input your DDNS account information since it’s your gateway to the internet.
Thanks,
Jasbryan
Cisco Support Engineer
.:|:.:|:.