How does a forest-dwelling dhutanga practitioner get food? According to "The Bhikkhus' Rules: A Guide for Laypeople" by Bhikkhu Ariyesako:


  The Buddha suggested that the basic source of food for bhikkhus was
  that received on the morning alms round (pi.n.dapaata). This daily
  dependence on alms food reminds both the bhikkhus and the lay devotees
  of their interdependence and prevents the bhikkhu from becoming too
  isolated from the lay community. He 'meets' them every day and eats
  the food that they share with him. Several important rules are
  concerned with this as well as a major section of the Sekhiya Training
  rules.


On this page, āraññika dhutaṅga refers to:


  "the one who has the habit to dwell in a country side lodging remote
  from villages", that is to say the one who lives remote from inhabited
  areas.


And the ukkaṭṭha āraññika, the noble practitioner of the āraññika dhutaṅga is defined as:


  The bhikkhu who is a noble practitioner of the āraññika dhutaṅga is
  resolved to dwell and dwell into a forest monastery (remote from
  inhabited areas) a all the time, during the three seasons of the year
  that are summer (from March to June), the monsoon (approximately from
  July to October) and winter (approximately from November to February).


The "The Bhikkhus' Rules: A Guide for Laypeople" also states:


  In the Buddha's time some ladies were ambushed and raped on their way
  to give food to bhikkhus living in a dangerous jungle area. Their
  family criticized the bhikkhus for not warning them of the hazards. If
  lay people intend to give food to a bhikkhu(s) in such a danger zone
  then they must announce that to the bhikkhu(s) beforehand so that the
  bhikkhu(s) has a chance to warn them or reduce the threat.
  
  The rule can be summarized: "Eating an unannounced gift of staple or
  non-staple food, after accepting it in a dangerous wilderness abode
  when one is not ill is [an offence of Acknowledgement.]"
  (Paatidesaniya 4; BMC p.488)


So, how does an ukkaṭṭha āraññika get his food? Only from lay people who dare enter the potentially dangerous forest area to give them food or can they eat food that they can find in the forest like fruits?

If they eat fruits etc. in the forest, wouldn't that be a violation of another rule requiring the monk to get his food from alms?

Here, I assume that an ukkaṭṭha āraññika does not go to inhabited areas for alms, by the very definition of "ukkaṭṭha āraññika".
The page you linked for āraññika dhutaṅga seems to already have an answer:

A &quot;forest monastery&quot; is a monastery situated at least 500 (curved) bows lengths from the doorstep of a house situated on the remotest spot of a village.

This sounds like only a few miles away, so totally within walkable distance.

The bhikkhu who practises the āraññika dhutaṅga must fulfil several daily duties, such as performing his food collection by means of his bowl.

...

When the time has come to go collecting his food, the bhikkhu conveniently closes his robe, takes his bowl and proceeds to a neighbouring village in order to receive his daily meal. Before proceeding to the village, he puts into order all that which remains outside of his lodging and properly shuts down the door. If the path leading to the village is rough, he can tread it with his feet dressed in sandals. However, he must remove them and set them aside just before crossing the entrance of the village. He must minutely observe all sekhiyas. He enters in houses with attention and manages remembering his own location (in order to trace back the house of a dāyaka who invites him to come collecting his food at his home), he avoids entering while not collecting food or in case of emergency. He avoids being too slow or too fast while entering and going out of houses. When he waits standing with his bowl, he stands still neither too near, nor to far from the house. He never waits too long in front of a house. When his collection is completed, he doesn't go to people's house, without good reason. He doesn't waste his time into idle talk. He enters his forest monastery without delaying.
When he enters back his forest monastery, he conveniently performs all allotted domestic duties, such as the storage of drinking water for various uses, or sweeping. When darkness already came down, he lights up some oil lamps. So as to avoid any danger caused by animals, he catches up a stick for protecting himself. He tries to trace back the location of the stars and the four directions (South, East, North, and West) so as not lose himself. He pays attention to the calendar (regarding uposatha days, etc.)
If a bhikkhu who lives in a forest monastery has an unhealthy instructor or preceptor and the monastery doesn't receive any medicines (or medical materials likely to heal the later), he must take him to a village monastery, where he will easily receive all necessary cares. By doing so, the bhikkhu who practises the āraññika dhutaṅga should leave the village monastery before dawn so that he will have reached the forest at dawn time. If the state of health of his instructor or preceptor has worsened, he must remain in the village monastery for the sole task of looking after him. In this case, no worries for his dhutaṅga. The state of mind being the main element, the dhutaṅga is not broken.

So it sounds like visiting the village is allowed (and even required!) for food collection, and staying in the village overnight is allowed for emergencies.