Repentance and expiation are required from one who commits zina during the day in Ramadan

We praise Allah, may He be
exalted, for having guided you to repent from this grave sin. Whoever
violates the sanctity of the sacred month by committing the sin of zina,
repentance is a blessing from Allah, may He be exalted, that calls for
gratitude and appreciation. The greatest act of gratitude for it is to
adhere to it and persist in it, to continually ask for forgiveness and to
regret what has happened in the past, as well as asking Allah, may He be
exalted, in all places and at all times when supplication is answered, for
pardon and forgiveness, in the hope that Allah will accept his repentance
and forgive what has happened in the past. Allah, may He be exalted, says
(interpretation of the meaning):

“O you who believe! Turn to
Allah with sincere repentance! It may be that your Lord will remit from you
your sins, and admit you into Gardens under which rivers flow (Paradise) the
Day that Allah will not disgrace the Prophet (Muhammad SAW) and those who
believe with him, their Light will run forward before them and with (their
Records Books of deeds) in their right hands they will say: “Our Lord! Keep
perfect our Light for us (and do not put it off till we cross over the Sirat
(a slippery bridge over Hell) safely) and grant us forgiveness. Verily, You
are Able to do all things”

[at-Tahreem 66:8]. 

With regard to expiation, it is
obligatory for you, according to the consensus of the fuqaha’ of the four
madhhabs. They have stated that expiation for intercourse during the day in
Ramadan is obligatory in cases such as yours, whether that intercourse took
place within the framework of a permissible relationship or a forbidden
relationship. If it took place within the framework of a forbidden
relationship, then the sin of immoral conduct and transgressing the limit
set by Allah is added to the sin of violating the sanctity of the fast and
the sacred month. 

Al-‘Imraani (may Allah have
mercy on him) said: 

If he commits zina with a woman
during the day in Ramadan… he has broken the fast [and he stated that
expiation is obligatory].

End quote from al-Bayaan fi
Madhhab al-Imam ash-Shaafa‘i (3/525). 

The scholars of the Standing
Committee for Issuing Fatwas said: 

The Prophet (blessings and peace
of Allah be upon him) stated the ruling that expiation was obligatory for a
Bedouin because he had intercourse with his wife deliberately during the day
in Ramadan when he was fasting. That was an explanation on his part
(blessings and peace of Allah be upon him) of the reason for the ruling. The
fuqaha’ are unanimously agreed that the fact that the story refers to the
Bedouin having had intercourse with his wife implies that expiation is also
required in the case of intercourse with a slave woman and in the case of
zina. They are also unanimously agreed that the fact that the one who had
intercourse came and expressed regret does not have any impact on the
obligation of offering expiation.

End quote from Fataawa al-Lajnah
ad-Daa’imah (10/300) 

‘Abd al-‘Azeez ibn Baaz, ‘Abd
ar-Razzaaq ‘Afeefi, ‘Abdullah ibn Ghadyaan 

Finally, we advise you to fear
Allah, may He be exalted, in private and in public, and to repent from
having not prayed and fasted in the past, and to strive hard to make up what
you have missed of those actions at the earliest opportunity. 

For information on expiation and
the rulings thereon, please see the following questions:
1672, 22938, 38023 and
39734 

And Allah knows best.