Thank you for all your input, I got someone to solder a 14V one in the capacitor's place. The router works after the replacement.
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Yep, cap blew up... I thought at first you were talking about just the heat shrink lable came off somehow, but no, that's the classic short or breakdown, and steam pressure blows the entire aluminum can off. They either do that, or the can splits on the end, where they have scoring to encourage a split, vs the high velocity can launching.
I would carefully check all the other electrolytic caps for swelling, evidence of heating, i.e. the heatshrink coating splitting or such... and replace them with good quality ones. At least equal to higher than the original voltage rating. And, of course, electrolytics are polarized, be careful to get the + and - terminals in the right place on the board! Or, you will soon hear another bang or two...
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An electrolytic capacitor is a sealed unit containing aluminum foil wrapped with insulating paper and flooded with an electrolyte - a liquid that provides the environment necessary for the aluminum foil plates to do their jobs of storing electrical charge. The electrolyte (hence the name of the component) is a corrosive acidic liquid. If it escapes the casing (i.e. the 'cover') it will damage other metallic things it contacts.
Sometimes electrolytic caps fail by bursting, or becoming pressurized and leaking through a pressure-release hole. The outer cover is made of aluminum and has score grooves in the top which are there to permit it to 'burst' in a way that's easily visible. Only in extreme cases would one blow the entire casing off. That's a bad, bad fail.
The aluminum 'cover' is not simply a decorative cover, it is absolutely necessary for the capacitor to work, and to keep the liquid inside. (not to be confused with the plastic shrink-wrapped label that surrounds the aluminum casing. That plastic sleeve serves only as a label.)
The one in your picture has clearly failed in a spectactular way.
Not only is the released liquid a hazard to the other components nearby, the capacitor is no longer able to do its job of helping to smooth the voltage to the device, so it will operate erratically if at all.
There are plenty of comments here with the proper advice: Either replace the whole device (the router), or at the very least clean the spilled electrolyte from the board and replace all caps of the same type/brand with new ones (hopefully of better quality).
The fact that this cap failed in this way is an earmark of very poor quality on the manufacturer's part. Be suspicious of other devices from them.
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Thank you so much for going into detail, that really helps. I've got only the damaged capacitor replaced (with a 14V one, as opposed to the 10V that failed) for now, but I'll look into getting the rest of them checked and replaced with new ones of better quality. It's a shame Xiaomi opted for low quality capacitors, especially when they're quite cheap.
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Overheating and falling out of the board is a possibility. A better photo of the capacitor and motherboard would help. Blurry, can't see the leads. The picture of the motherboard, location C3: insufficient solder? The cap failure mode is usually the capacitor swells up on the top side and either bursts or short circuits while remaining soldered onto the board causing other parts to fail. The capacitor is polarized, so get an electronics technician to solder in your replacement part. The other two electrolytic capacitors could probably use inspection and touch up soldering.
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