The Hamburg international airport (HAM) is well connected to all major destinations in Europe. Conference accommodation and CFEL can be reached by public transport in 30 and 60 minutes.
Central train station and Hamburg Altona are connected to the German high-speed train-network offered by https://www.bahn.com/en/view/index.shtml
Long-distance lines destinate close to central station and are linked to the local public transport. International connections are for example offered by FlixBus https://www.flixbus.co.uk
You either take the S1 } to Barmbek and then change to U3 in direction Wandsbek-Gartenstadt to Feldstra\ss e. In this case you'll have to walk about 500 m to the hotel.
Or take the S1 to Reeperbahn and walk 1 km to the hotel (and see parts of Hamburg's famous red-light and party district)
Take the U3 in direction Barmbek to Feldstraße and walk the remaining 500 m.
The most convenient way is to take the S1 to Altona and conitue by bus number 2 to Luruper Chaussee (DESY).
You either take the bus number 3 in direction Schenefeld or Stadionstraße S21 to Luruper Chaussee (DESY).
or first take S21 or S31 to Holstenstraße and then change to bus number 3 to Luruper Chaussee (DESY).