After the PIN constraint was removed from the Huawei stick, the TP-Link 3G Router (TL-MR3420) is working fine in WAN preferred mode. Soon after I disconnected the connection to the DSL uplink, the router began setting up the Huawei E1550 stick.
This is working so flawless, I am pretty sure my parents will no longer complain about the unstable DSL link, 3G will just take over, in case it’s not working and the limited bandwidth will still feel ok on the single device, that is connected.
By the way not only the failover but also the giveback is working fine, I re-connected the WAN uplink and after half a minute, the IP has returned to the DSL provider.
Using ping to a single external IP, the failover took a break of 8 packets (8 seconds), the giveback 2 packets. Comparison in this test is not really fair, the UMTS stick did the full handshake whereas the WAN uplink just a DHCP request, since my DSL router on the WAN endpoint was constantly connected.