I have a similar problem that started last week after an update was applied to my Centro Piccolo router by Swisscom. I would describe it as:
Swisscom router firmware update has compromised DynDNS and/or NAT Loopback.
Until the update last week I was able to connect from within my LAN to my SSH server via the external DynDNS address. After the update, I no longer can. There seems to be two possible causes, both caused by the update.
1. The DynDNS website reported that my router was no longer running the DynDNS client. I looked at the DynDNS settings in my router and found that they had been wiped, so I re-entered them. However they didn't work: the DynDNS status on the Overview page of my Router Configuration said "DynDNS Failed". I looked at the log and noticed that the DynDNS domain I had entered was being incorrectly pre-pended to the DynDNS domain. On closer inspection, I found that it's no longer possible to enter my DynDNS domain in the router. The firmware update that Swisscom did obviously wiped it because it considers my DynDNS domain no longer valid. A quick look at DynDNS' website shows that the SynDNS domain I was using (dynalias.net) is no longer available to new customers. Obviously Swisscom has noticed this change and have decided without warning (unlike DynDNS who still support these legacy addresses) to prevent the use of legacy DynDNS addresses. So I changed my domain at DynDNS to one that's still supported and that corrected the first problem.
2. My router System Log contains the following errors, numerous times (the first at least every 5 minutes):
L5 sdb[242]: DNS: could not rename /var/tmp/hosts.hostdb.new to /var/etc/hosts.dyn
L5 upnpd[1366]: SUBSCRIBE not implemented yet
I don't know if they are related to each other but the first one clearly is related to DynDNS in some way and only started when the firmware update happened.
I called Swisscom support and asked them what they had changed. After trying to talk me through a reset of my DNS settings (until I explained that I didn't have a DNS problem, only a DynDNS problem), the guy told me that they changed nothing to do with DynDNS - untrue since a) my settings had been wiped, and b) could not be re-entered. I asked for assistance with the error messages above and was told that I had to call a chargeable support line for that as Swisscom doesn't support DynDNS. Not sure what the latter statement is supposed to mean - but it strikes me as rather odd that Swisscom won't resolve an error message in their own router.
So my current situation is that my DynDNS is now working: I can PING my DynDNS address from my LAN, and I can TELNET to it. But I still cannot SSH to it. I know that my SSH server is still working because I can still access it using the internal IP/DNS name. Something in the recent firmware update (reported on the Overview page as 9.0.7h0d57) seems to have compromised the DynDNS and/or NAT Loopback on which I was relying.