Gosh a pp has suggested caustic soda and boiling water. Definitely do not do this! Adding water to caustic soda causes a chemical reaction that can make it explode and it is highly dangerous and will burn you and likely blind you if it gets in your eyes. Adding caustic soda to water is safer but it's still nasty stuff and best avoided unless you know what you're doing and have the proper safety equipment.
Soda crystals are completely different
So long as you follow the instructions on the packet of caustic soda you're fine. The packet we have here says to add it to cold water (it makes heat as it dissolves); to pour it into the drains; then rinse it away with copious amounts of hot water. So yes, you do use hot water with caustic soda, but you don't mix the two directly.
It is nasty stuff, it is different to soda crystals, but it does shift stubborn blockages which have defeated other methods so it's worth considering.
Problem with chemicals is so,etimes they just move the blockage further along the sink.
They don't often. They usually break up the blockage and disperse it. Physical methodsdrain rods and so onare far more likely to just force a blockage along. But again, usually, they break up or penetrate the blockage then the flow of water through the pipes breaks it up more.
ALso... all the people recommending buying specialist equipment and leveling up manhole covers- seriously?
This doesn't seem unreasonable to me at all! We've done all sorts to unblock drains, both at home and at our parents' and friends' houses. It's not difficult if you're prepared to give it a go, and it's very difficult to cause any damage this way. You do have to be careful with lifting access covers as they're heavy and can cause serious damage to hands if you don't use proper lifting tools. But it's not complicated or difficult.