Brian Cipenga had fired the Congolese into a shock lead after seven minutes in Atlanta, but Harry Kane scored twice in the second half of a nervy contest to send England into a last-16 meeting with tournament co-hosts Mexico in the Estadio Azteca.
"We kept believing. We had the worst start possible. First shot, first goal, Then, it became even more difficult," said Tuchel.
Kane was denied a first-half penalty after it appeared DR Congo goalkeeper Lionel Mpasi had tripped him.
"After the first water break, we were on top of the game. I think we should have had a penalty," Tuchel added.
"The substitutes came on, and put the effort in, and we won it. Well deserved, but we had to work a lot.
"We have to have that mindset if it is getting hard, it is getting hard, but don't lose patience and don't lose belief."
Of Kane, Tuchel said admiringly: "He is getting better."
FIFA World Cup 2026
The 2026 World Cup took place from June 11th to July 19th in the United States, Canada and Mexico. The tournament featured 48 national teams and was won by Spain, who beat Argentina in the final.
Full schedule and results | Group standings and bracket | Past winners of the tournament | Our team of the tournament | About the 2030 World Cup