After researching the entire internet - except for chinese and cyrillic pages - I decided to implement some brute force solution. I then included a soft watchdog, hand coded, adapted from the Andreas Spiess blog and returned to the old router to test. The new version is working consistently for 16 days and counting. The log shows that the WD kicked 5 times, the first time at 28min, the second at nearly 3 hours. Curiously the WD has bite after a greater interval each time. The last time about 7 days ago and after that it did not show up. Don't ask me why: I don't have a clue for such behavior.
The soft WD keeps the I/O status and context intact in a way hat the software reset becames transparent for the application.