Ipv6 bad routing? Turris can't directly access upstream DNS server via ipv6 but windows can when connected the same way

all mobile networks in Germany offer IPv6 and PD.
Your modem needs to be configured. Some may have issues.
As shown here Folie 68 the “PDP” settings need adjustment.
So yes, all (German) Mobile Provider do provide IPv6 and PD.
To reach someone on the PD address (termination on the Mobile) often fails: Carrier grade NAT and similar f*ups ruin the IPv6 connection. In principle it works (I often tried successful). In Germany best results are with DTAG/D1/Telekom. For plenty of money they offer a “fixed IPv6” on the mobile that can be reached (terminated on) all time. So yes, Mobile should work like any Internet device.

Turris refusing to ping an address that I can always ssh to was a problem, too. Sometimes it worked for a few minites after reboot of Turris Omnia. A complete new setup from scratch eventually resolved this issue. Not understood, afraid it might come back ping stops working.