Yes this battery should fit in that specific model

Normale Batterien

I do not know type SMT1500RMI2U. So I cannot say anything

No, this is a 12V 7Ah battery (APC specifies 84VAh).

APC Back-UPS Pro 550 ist laut Modell Nummer ein BR550GI https://static.digitecgalaxus.ch/Files/1/3/8/4/4/8/7/DFAH-84RL3T_R1_EN.pdf

Take it to one of the well-known battery collection points (Migros, Coop, etc.). In my case, it was accepted without any problems. Otherwise, take it to a computer shop near you, where it will then be passed on to SWICO.