CoCo is a high impedance antenna, higher than 75 ohm input impedance of DVB-T, causing impedance mismatch.
Incidently, the impedance "seen" by the DVB-T dongle depends on length of coax between Antenna and DVB-T.
As the length of coax is increased/decreased, the impedance "seen" by DVB-T gradually varies from antenna's impedance to a lower value and then back to antenna's impedance in a cyclic manner (see second image below). This cycle repeats every 1/2 wavelength. There is one point in every 1/2 wavelength section of feed coax which gives lower value of impedance, and hence better performance. The last half wavelength section at DVB-T also has one such point, and if feed coax is chopped to that point, impedance matching improves.
Chop the end of feed coax at DVB-T end, in 10mm steps for say 5 to 6 steps. After each chop, run your system for a while and note what maximum range you get. It is very likely that you will re-gain your lost mileage
Please note that success is not guaranteed, as it depends on exact impedances of antenna & coax. However since this is an easy method, there is no harm in trying.
At each step you will have to redo the end connector. If feed coax's end connector is twist-on F-male type
it is very easy to remake with each trim, and you can use the same F connector again and again. If connector is crimp type, it is more effort and you will waste many connectors.