Hi @Jan,
ETH sent via the Arbitrum network can’t be seen in Trezor Suite as this network is not natively supported in Trezor Suite. You need to use 3rd party app MetaMask, information on how to connect Trezor to MetaMask: How to use Trezor with MetaMask
Then you need to add the Arbitrum network which is so-called EVM (Ethereum Virtual Machine) chain. Example on how to add EVM to MetaMask (with a different EVM though): How to use EVM compatible chains with Trezor (Matic, Avax)
(you need to find “Arbitrum One” on chainlist.org and add this network)