B12 is absorbed in the small intestine.

Milk is curdled in the stomach by the acid and becomes a solid (like cottage cheese).

The fat content of the milk will determine how fast it leaves the stomach. The higher the fat, the slower it moves.
I don't expect much to be absorbed, sublingually.

Find some time of day when you are hungry and haven't eaten for several hours. Say before lunch? Before dinner?

To use oral B12 correctly so you will absorb it, you have to find a window where nothing is present to absorb it. (like a sponge).