It's not what Signal is built for. If you want to join public chat rooms, you can use Telegram equally securely. E2EE never hurts in the case of public groups, but since you're not able to verify none of the group members are malicious, and logging and leaking your (and everyone else's) comments to every bad actor, it doesn't matter.

Your only protection is anonymity, but that's something not even Telegram offers by default: This has lead to speculations about attacks where the Chinese cyber army has performed massive reverse look-ups of Hong Kong citizens' phone numbers, by storing what TelCo has in it's database / what IMSI catchers / hacked towers see, and adding them as contacts. This by default reveals Telegram user's username to the attacker. You can disable this by opting in, but very few do.