Possible solutions are very nicely listed in this article. Basically few options:

1. Send small amount of funds to the 20address manually to keep things going
2. Use a HD wallet like electrum that allow for a higher gap limit setting
3. Switch to services like blockonomics that automatically send funds to the 20th addresses allowing merchant to keep on generating receiving addresses without any interruption

Possible solutions are very nicely listed in this article. Basically few options:

1. Send small amount of funds to the 20th bitcoin address manually to keep things going
2. Use a HD wallet like electrum that allows for a higher gap limit setting
3. Switch to services like blockonomics that automatically send funds to the 20th addresses allowing merchant to keep on generating receiving addresses without any interruption