I’m not sure how long the current antenna is, but a longer antenna for DAB+ will more than likely actually make reception worse.
Big mistake often made, I even find myself doing it often, is thinking a bigger(longer) antenna will give better reception because the bigger the antenna, the more signal it’s going to pick up right, not always, if you’re in a good or reasonably good signal area, you want the antenna to ideally be half wavelength, (in length) which in the case of DAB+ in Sydney would be 725cm long for a whip style antenna, such as on a portable radio, the other half of the antenna is internal to the radio to make a dipole, but the internal half isn’t adjustable & is probably not the ideal length anyway for a true dipole, but it helps.
If you make the antenna length too short (shorter than half wavelength), it’s not going to pickup all the signal power, so you’ll get a weak signal, as you start to make it longer than half wavelength, you start to pickup the wave as it’s crest is changing from full positive to the trough at full negative (or vice versa) & the power in the wave will actually start to cancel itself out, giving you a weaker signal strength.
Antennas on portable radios are generally made of a length more suitable for FM, as you can always make it shorter for higher frequency stations, but even then, they’re mostly under length, as for half wavelength of an FM frequency, the antenna would be too long & unwieldy for a small portable radio at 1.5m long (half wavelength) for middle of the band 98.1MHz frequency.
At work, directly under the tower some of the radios we use for monitoring the FM & DAB+ (particularly), even half wavelength is too long & we’ll get better DAB+ reception with the antenna length at 30 - 40cm long (roughly 1/4 wavelength).
Do some testing with the antenna at short/er lengths & see if the signal is better in those areas where you had marginal reception.
Disclaimer: Personal opinion, not the opinion of my employer (TXA), I do know/note there are still quite a few areas around Sydney where there is weak or no DAB+ reception, no matter how hard you try on a portable radio, honestly to cover the Sydney licence area (forget overspill areas) to FM equivalent coverage on DAB+ we’d probably need to install another 4 or 5 in-fill translators, & currently there’s not enough listeners to make it financially viable.