"I swapped tokens for DOT on PancakeSwap, but now they don't show up in my wallet." 

A DEX is a decentralized exchange, such as PancakeSwap for example, which runs on the Binance Smart Chain (BSC).

Polkadot is an entirely different network from other chains such as Binance and Ethereum. So if you use DOT on PancakeSwap, you're not actually using the real DOT. What you are trading there is a Binance-pegged version of DOT on the Binance Smart Chain. 

Therefore, if you need any troubleshooting help, please contact the Customer Support of PancakeSwap or Binance. Unfortunately we cannot assist you, since no transaction took place on the Polkadot network.

The actual DOT - the native token of Polkadot - can only be transferred from/to valid DOT addresses on the Polkadot network, which all start with 1, not with 0x.