Huawei E3372h: ModemManager wrongly claims that IPv6 is not supported

I have a Huawei E3372h modem, reflashed to "stick" firmware (i.e. downgraded in order to avoid IPv4 NAT):

Manufacturer: huawei Model: E3372 Revision: 21.200.07.00.00

It is supported using the huawei_cdc_ncm kernel module.

My mobile operator (MTS Russia) announced support for IPv6 some time ago. However, ModemManager cannot use it:

Couldn't connect bearer: 'Only IPv4 is supported by this modem'

Which is true out of the box, but becomes false when one pokes into NVRAM and replugs the modem:

AT^NVWREX=8514,0,4,01,04,00,00

This setting persists after unplugging the modem.

Here is now to connect with IPv6:

echo -en 'AT^NDISDUP=1,1\r\n' > /dev/ttyUSB1
dhcpcd wwp0s20u2i2

There is also an AT^DHCPV6? command, but I have not yet found how to use it correctly.

Connection log:

wwp0s20u2i2: waiting for carrier
wwp0s20u2i2: carrier acquired
DUID 00:04:af:6d:df:b9:6c:72:11:e4:b5:7f:68:f7:28:2c:d1:e0
wwp0s20u2i2: IAID 10:1f:00:00
wwp0s20u2i2: adding address fe80::41b6:aeb:e025:1034
wwp0s20u2i2: soliciting an IPv6 router
wwp0s20u2i2: rebinding lease of 10.142.0.102
wwp0s20u2i2: probing address 10.142.0.102/30
wwp0s20u2i2: Router Advertisement from fe80::4e54:99ff:fe45:e5d5
wwp0s20u2i2: adding address 2a00:1fa2:4404:a944:2d22:4847:88a9:e1a8/64
wwp0s20u2i2: adding default route via fe80::4e54:99ff:fe45:e5d5
wwp0s20u2i2: soliciting a DHCPv6 lease
wwp0s20u2i2: leased 10.142.0.102 for 518400 seconds
wwp0s20u2i2: adding route to 10.142.0.100/30
wwp0s20u2i2: adding default route via 10.142.0.101
forked to background, child pid 4651

dhclient -6 does not work, because this is based on (solicited) router advertisement, not DHCPv6.