ADSL modems often DO contain a router, though that's not always obvious to users. I'm wondering if your broadband provider may have pushed a firmware update to your router and it's screwing something up.
You said your Obihai device was plugged into the ADSL modem directly, which is good, but if you can access the router configuration
in the DSL modem (not your separate external router, assuming you have one), can you try temporarily putting your device in the DMZ?
I'll again suggest that you contact
support@obihai.com but in addition to the information I said you should provide in the previous post, I'd also mention the manufacturer and model number of your ADSL modem, and your broadband provider's name. Chances are that if your provider did push bad firmware, customer support has already encountered this issue and may have a fix or workaround.