@Gronis 

So there is absolutely nothing curious in the logs?  If not, my guess would be a power supply issue.

  @Gronis 

Ok, so the OC200 version looks like:

Sounds like you are running your OC200 via POE..which means you must have a POE switch in your setup as well?

I would keep the 5.7.6 firmware on the OC200 for now, forget and then re-adopt the ER7206.  If that doesn't work, you can try factory defaulting the OC200 (take a backup first) and rebuild a minimal config to get you back up and running again (ie WAN settings, LAN settings, leave ACLs, port forwards for now...this is just a stability test), then factory reset the ER7206 and do a clean adoption.

Since the ER7206 has an AC input, if there is a PSU issue it will be internal and not user fixable.  The only suggestion I have here is bring power from another circuit somewhere with an extension cord in case there is some 'noise' on the current plug/circuit (like a big motor starting up (fridge, freezer, furnace...) or a welder or....??)

The quickest fix may be just to purchase a new 7206 if you are unable to resolve by defaulting and rebuilding the soft configuration.

  @Gronis 

Well, it sounds like a hardware fault then.  Recommend you open a support ticket with TPlink to confirm.

  @Gronis 

Looks like the ER7206 comes with a Limited Lifetime Warranty.  That at least is good news.