The two summons that are available for the Dragonslayer Armour are not appearing. The fan site does not mention that any quest must be completed. One is supposed to be in the Shrine (physical - Sirris), but there are only the main NPCs. Where are the two NPCs and how to make their summon signs to appear? I am Embered when looking for them in the Lothric Castle and it is "The Fire Fades" (GOTY) Edition. Edit: it looks like one od them has a quest line, to rescue someone, but what about the other one?
There are only two summons available for this fight.
Eygon of Carin can be summoned, provided you haven't killed him, or the maiden. His Summon Sign can be found in the building to the right of the stairway leading to the boss fog on the right side just beyond the doorway.
The other Summon Sign belongs to Sirris, who, as you mentioned, has a backstory leading up to this event. You will need to have followed her questline up to defeating Creighton the Wanderer, for her sign to appear.