I think that is a normal relay, and not a flasher relay. Hella seem to have loads of different relays and I can't find that exact one. The part number is probably 4RD 003 620-13, as that is the form of the Hella part numbers. The fact that is says 20 / 30 Amp and that the part number starts "4R" would point to it being a standard relay.
Well all that just means that the flashing function and the current sensing is not done in that relay, so there is no point in changing it. I had hoped that the flasher relay would say something like "2 x 21W" and could be changed to something that could handle more bulbs.
What I would do is to test that all the wiring is OK by plugging in the trailer, and taking out the indicator bulbs. If the flashing doesn't work on the bike with the trailer plugged in and no indicator bulbs in the trailer, you have a wiring fault. You should also check with a voltmeter that you are getting 12 V, turning on and off with the indicators, at the trailer indicator bulb holders.
Assuming that is OK, the easiest solution is to find some LED bulbs for the trailer indicators. They won't take enough current for the bike to notice.
If you don't want to do that, you need to get two automotive relays, and connect each as follows:-
Pin 85 - Ground
Pin 86 - indicator bulb on bike
Pin 30 - Battery on bike (via a fuse)
Pin 87 - indicator connection on socket
You need to do that twice, once for each side of the bike. You only need one fuse, as long as you connect both relays to it. The fuse should be close to where you get the 12 V from.
The idea is that when the indicators operate, the relay for that side of the bike operates as well. The current needed to operate the coil is quite small so that should not be noticed by the bike's electrics. The contacts on the relay allow current straight from the battery to the indicators on the trailer.