Nobody claimed the he was connected to an Islamic Terrorist network like you said. But they have found that he was likely Isis inspired from the home made Isis flag found in his home. Isis did claim it, but we don't know if its true. But looks like he was Isis inspired.
Hand-painted ISIS flag found in room of Bavaria train attacker, terror group claims responsibility.
Police discovered a hand-painted Islamic State flag in the room of the Afghan teen who attacked passengers on a train in Germany. The group claimed responsibility, but the Bavarian interior minister said evidence indicates the teen could be self-radicalized.
Islamic State (IS, formerly ISIS/ISIL) has said the attacker was one of its fighters, according to IS-affiliated Amaq news agency.
Police have also found documents suggesting the Afghan teen who attacked passengers on the train in Wurzburg had links with IS.
“It is clear that apart from Islamic State terror group’s flag, several documents that indicate of the suspect's links to IS have been discovered in the suspect’s room,” N24 reported, adding that the documents have been taken by police for further analysis.
Interior Minister Joachim Herrmann said Tuesday that a note was found in the teen’s room indicating he may have been self-radicalized. In the note the attacker wrote of Islam and a “need to resist.”
“We also discovered a text in Pashto written partly in Arabic and partly in Latin, which indicates that this could be someone who has self-radicalized recently,” Herrmann said, noting that this had yet to be proven.
Herrmann also said that people who had been close to the attacker told investigators he had an appearance of a calm, not expressly religious person.
The official stressed that the attack is no reason for generalized suspicion of refugees. He added that attacks like the one in Wurzburg would not stop Germans from riding in trains and that people must not let “such attackers” ruin their lives.
https://www.rt.com/news/352015-isis-flag-bavaria-attacker/
