I've never seen 𢧜 in any other character before, so I'll take that 𢧜 is exclusively for 鐵 (just like 邊 and 微 which I've asked before in the past). So where does 𢧜 come from? It seems to be a combination of relatively unrelated components, and none of them sound like 鐵.
「鐵」 (Zhengzhang OC: /*l̥ʰiːɡ/, black metal > iron) is comprised of semantic 「金」 (metal) and phonetic 「𢧤」 (/*l'iɡ/, using the reconstruction of 「秩」)*. 「𢧤」 (top-left component 「大」) was later corrupted into 「𢧜」 (top-left component 「十」).
「鐵」 represents the same word as 「驖」 (black horse), which also uses the component 「𢧤」. If you recursively trace the components downwards, you eventually end up with a phonetic 「呈」 (/*rleŋ/).