Every day at around 10 a.m., Anderson Lucena grabs his phone and goes online, hoping to get a life-changing appointment.

He opens CBP One, an app created recently by the U.S. government that schedules appointments at border posts for migrants who want to seek asylum in the U.S.

Then he selects a border crossing and presses a blue button, hoping for the best. The app is only awarding about 1,000 appointments for migrants each day, which is way below the current demand.

"Only God knows when this will work," said Lucena, who comes from Venezuela and has been stranded in Mexico City for three weeks. He's been staying at CAFEMIN, a shelter run by the Josephine sisters in Mexico City. "I don't want to risk being sent to the south of Mexico," he said.

On May 11, the United States changed rules for asylum-seekers following the end of the enforcement of a provision of the U.S. health code known as Title 42 during the COVID-19 pandemic.

Now migrants who want to seek asylum in the U.S. are being discouraged from crossing the border on foot, and are instead being told to schedule appointments on the CBP One app. Those who cross the border without permission and are turned back, face a five year ban on reentering the country.

The changes have reduced the number of illegal border crossings by around 50%, according to the Department of Homeland Security. But they're also putting pressure on shelters in Mexico, where thousands of migrants like Lucena are stranded, as they try to schedule appointments on the government's app.

Recently, shelters in Mexico City, which is 620 miles away from the nearest border crossing, raised alerts about the humanitarian situation, urging authorities in the United States and Mexico to do more to protect stranded migrants.

"I'm worried about the emotional state of these people, their anguish and their desperation," Sister Magdalena Silva Rentería, the director of the CAFEMIN shelter, said in a statement posted on Twitter. "This situation is pushing us to the limit, and our shelters are on the verge of collapse when it comes to providing enough water, food, psychological support."

CAFEMIN -- which stands for Casa de Acogida y Formación para Mujeres y Familias Migrante (Shelter and Training Center for Migrant Women and Families) -- was founded in 2012 and initially designed for up to 100 people. For the past three weeks, it has been hosting around 500 migrants each night. Beds have run out, so most of the migrants sleep on gym mats that have been laid out on the floor of the shelter's roofed basketball court.

Hosting so many people is exhausting, said CAFEMIN's outreach coordinator, Mario Monroy. He mentioned that the shelters had to turn people away, or limit how many weeks others can stay.

"We want to help," he said. "But many times we feel like we're between a rock and a hard place."

Monroy said the situation has been critical since October of last year, when U.S. officials started to send thousands of Venezuelan migrants into Mexico, following a deal between both countries.

"Mexican immigration officials were bringing people from the border on buses, and leaving them at our shelter's doorstep," Monroy said. "These were people that had been deprived of their liberty," after being held in detention centers in the United States, "had been mistreated, and they had no information on what to do next."

Civil society organizations and religious groups have called on Mexico City's government to open up new shelters where migrants can stay and receive orientation while they seek appointments at the U.S. border.

In a recent editorial, published in its weekly journal, Desde la fe, the Archdiocese of Mexico City said there are currently five shelters in the city that receive support from the Catholic Church. But it argued that this was not enough to cope with the large numbers of migrants arriving in the city.

"We call on authorities to revise their policies towards migrants," the editorial said. "So that the rights of the most vulnerable and their protection are guaranteed."

Mexico City's municipal government recently opened a shelter on the outskirts of the city, but it is located in an area that lacks public transportation, and where it's hard for migrants to find temporary jobs.

At the CAFEMIN shelter, Lucena, who is a construction worker, said that he was worried about his family's future. He said returning to Venezuela, where the minimum wage is currently worth about $6 a month was not an option. So he will insist on going to the United States.

"I just want to buy a house for my children," he said, as his smallest kid played with a toy dinosaur. "I just want to get somewhere where I can work, save money and get ahead in life."