To add a custom network manually in the frontier extension, these are the steps you need to follow
Open Frontier Wallet on your web browser.
Navigate to the Settings tab.
Click on Manage Networks.
Select the + icon on the bottom-right corner.
Fill out the relevant information regarding the network you want to add - Network name, RPC URL, Chain ID, Currency symbol, and Block explorer URL.
If the information entered is valid and the network exists, you can continue by choosing if you want this network to be added to your current wallet or to all wallets.
The network is added to your wallet(s) based on the choice you made in the previous step. To confirm that the network has been added, check the list of networks on your Frontier extension.
There are no limits to the number of networks that can be added to the Frontier Wallet browser extension.
Manage, Transact, and, most importantly, have fun!