To build off of the previous answer from DarkCygnus, and hopefully provide more insight into the game's mechanics, for future situations:
When you play Return of the Dragon Lords, your monster is not immediately Summoned. Your opponent has an opportunity to respond to the activation of your card and chain a card or effect before it resolves.
When your Spell resolves, that's when the effect of any card actually happens.
Cards like Bottomless Trap Hole and Torrential Tribute don't negate Summons before they happen. They can be activated during what is known colloquially as the 'summon response window'-- the period immediately following a Summon.
During the summon response window, since the monster(s) in question have already been successfully summoned, their effects are now applied to the field.
This is not the case for cards that negate Summons, such as Solemn Warning, Solemn Strike, and a few others, since cards that negate Summons prevent the monster from ever reaching the field in the first place. In a case like this, even if RotDL was in your GY, it could not stop a card like Solemn Warning.
RotDL is also a rather weird card in some regards: its protection effect does not activate, since there is no ':' in its protection effect. Applying it does not count as activating a card effect, and does not start a chain. Even in a situation where your opponent can prevent you from activating cards or effects, you can still apply it without worry.
I'm sorry this doesn't necessarily answer your question, since it was already answered. I just hope what knowledge I've shared with you comes in handy at some point!