DNS is the lookup that turns a name into an address. Most domain blocking in Bangladesh happens right there, which is why changing the lookup fixes it — and why it is worth doing before anything heavier.
No. A VPN sends all your traffic through another country, which slows everything down and costs money. This changes one setting: who answers the question “what address is this name?” Your traffic still goes the same way, so speed is unaffected.
Android 9 and newer has this built in, no app required.
one.one.one.one and save.It applies to Wi-Fi and mobile data together, and it survives reboots.
iOS has no system-wide DNS field, so the free 1.1.1.1 app from the App Store is the practical route — install it and turn it on. To change it for one Wi-Fi network only: Settings → Wi-Fi → the ⓘ next to your network → Configure DNS → Manual → add 1.1.1.1 and 1.0.0.1.
1.1.1.1, Alternate 1.0.0.1. Save.System Settings → Network → your connection → Details → DNS → + → add 1.1.1.1 and 1.0.0.1, then remove the entries above them.
Setting it on the router covers every device on the Wi-Fi at once, including TVs and consoles that have no DNS setting of their own. Open your router page (usually 192.168.0.1 or 192.168.1.1), find DNS under WAN or Internet settings, and set 1.1.1.1 and 1.0.0.1. Write down what was there before, so you can put it back.
Google Public DNS is 8.8.8.8 / 8.8.4.4; Quad9 is 9.9.9.9. Any of the three works the same way — try another if the first does nothing.
Then the block is not at the DNS layer, and a DNS change cannot reach it. Open a different official address from the addresses page, and see how the deeper blocks work.