Frankfurt was in the borderlands of the Roman empire (which in 250 AD viewed Christianity suspiciously, but was more concerned with the "3rd century chaos" of multiple imperators than to be bothered with persecutions - until Diocletian)
This region was part of "Agri Decumates"
And remember that Christians were traveling all over - by 70 AD there were Christian communities already in what is now Spain, Portugal, the UK, Tunisia, Syria, Ethiopia, Yemen, Iran, India, uzbekistan and possibly even further afield. As to Europe - the Amber road had been in place for quite some time already - I think already 1000 years by 250 AD (need to cross check that) - during the 3rd century, Frankfurt sat right along the Limes Germanicus—the fortified frontier border of the Roman Empire. While Christianity was actively spreading through the Mediterranean, North Africa, and major urban hubs like Rome or Lyon, the northern military frontiers were still with very few Christians.
FInding a Christian community might be unusual but not impossible as Christians did travel or trade or fight there or were enslaved. the owner took a traditional, mainstream Roman practice—wearing a protective amulet—and completely Christianized the text inside it to invoke the protection of Jesus.
ConMind - he didn't live "before the Catholic Church" - the Catholic Church is mentioned in Acts 9:31 - ἡ ἐκκλησία καθ' ὅλης (he ekklesia kath'holes) and in the letters by pope Clement (the 4th Pope), by Ireneaus of Lyons, Ignatius of Antioch, Justin Martyr - all in the period 90 AD to 160 AD). Ignatius of Antioch used the term in writing around 110 AD. By the time this amulet was buried (mid-200s AD), church fathers like Cyprian of Carthage were heavily using the term "Catholic Church" to distinguish the mainstream Christian network from various regional sects or gnostic groups.
this artifact dates from c 250 AD - so ~200 years after the foundation of the Catholic Church at Pentecost in 33 AD
the church of the 200s AD was the early Catholic Church, sharing direct continuity in bishops, scripture, and sacraments.
btw, Co-Redemptrix is not official Catholic dogma and never has been
AND, ConMind - The oldest known written prayer to Mary, the Sub Tuum Praesidium ("Under your protection we seek refuge..."), was discovered on an Egyptian papyrus dating to around 250 AD - the exact same window of time as the German amulet.
The amulet is an exorcistic text - even today in the Catholic Church, formal exorcisms are strictly performed in the name and power of Jesus Christ - the name of Jesus has power