Cannot. M5250 has no storage sharing function.

If you don't have to access internet, it would be OK to connect MR3420 without any kinds of modem. MR3420 has a default Wi-Fi name which is printed on the bottom. You can use the laptop to detect it...

What's the firmware version? Is it the latest one? Did you ask ISP to check your line quality? When the connection is lost, how about ADSL light? Try DSL mode ADSL2 or G.dmt.

Cannot open attachments you provided. It is a synchronization issue and you can try to change DSL mode on web interface of W9980. Try ADSL2 or G.DMT. Wait about 2 minutes. If it is the same, you...

What's the region selected on RE200 page? Germany? Different regions have different regulations on transmisson power. You may choose US to make it operate as your expection but it is not compliant...

Try to input DNS server manually on router page(DHCP---DHCP Settings), 8.8.8.8.