The only advice I can give you is kindergarten stuff really - to physically take the dog out yourself after a meal - and watch the dog for the daily routine & anticipate the 'poo-time' so take the dog out in advamce.
Praise the dog when it does go outside, but don't go overboard punishing or shouting when it makes a mistake.
I have an 8yr old Basset, & he still has an accident here & there. I ignore it totally & just clean up, making him unhappy by shouting at him isn't going to make any difference & it's not his fault if I am asleep in bed when he wants to 'go'!
Not much help there, I'm afraid - but

& Good luck!