Usually reservation are not needed in Germany few exceptions like Nighttrains, Highspeed further to France or Eurocity´s to Poland & Denmark :)

If you miss a connection due the strike it´s the best to head to the “Service Point or DB Information” of the station (mostly on major stations) and ask them for help Often you get a Hotel Voucher and a letter that makes your Railpass valid for the next day (no need to waste another travelday of a Flexipass) or they provide you a taxi voucher (happend to me quite often Hamburg or Hannover to Berlin )

The reservation for Berlin - Warsaw can be exchanged at the “DB Reisezentrum” these are the International ticket desks at german station You can do it either in Mannheim,Frankfurt or even in Berlin :)

The “DB Information” in Mannheim is staffed till from 6am till midnight :/

@Cristian Martínez  please note that due the strike there will be no train from Berlin to Warsaw the Eurocities start from Rzepin (the first polish station)

my suggestion is to catch from Strausberg  a S-Bahn (atleast every 40mins due the strike) from Hbf or Ostbahnhof  and catch there the Regionaltrain to Küstrin/Kostzyn from there you have connection to Polish fast trains currently there is a short bus replacement on the last leg of the regional train from “Küstrin-Kietz” (last german station) to Kostzyn/Küstrin (final station in Poland) the bus just need 10minutes and from Küstrin you could catch a Intercity at 5pm directly to Warsaw :)

Thank you so much for the quick responses! If no reservation is needed on most German trains, how does it work? I just hop on and sit wherever I want? How does seating work? 

@Cristian Martínez  you can sit on any free seat above the seats or on the seats you found usually digital displays that show on which section the seat is reserved in older trains you have paper stripes next to the seats with the reservation informations