You said " Whatever I have learned so far in Advaita is that all the forms of God are illusory and that only Nirguna Brahman is the Original Supreme Personality of Godhead (I may be wrong). "
Yes. All forms of Gods are illusory but only for the realized (or at absolute level), as per Advaita. When living in this world for normal jivas, world doesn't become illusion all of a sudden. For example, you can't live without eating food though world is unreal in reality. One must do their dharma (duties) based on their respective Varna and ashrama, when they are sadhakas or in unrealized state. As per Advaita Vedanta, in reality world is illusion but here unrealized know that fact (world is unreal at absolute or paramarthika level) but still they feel world as real. After realization, they know or feel world as illusion including all forms of Gods.
To understand in detail, read According to ShankarAchArya in Advaita, is the universe “unreal” or “unworthy”? and World is real even according to Advait Vedanta?. World is real at Vyavaharika level and unreal at Paramarthika level, as per Adishankara Advaita.
Bhagavad gita 7.24 says ignorants don't know Brahman's highest state which is immutable and unsurpassable.
Here is what Adi Shankara says on this.
7.24. The foolish regard Me as the unmanifested coming into manifestation, knowing not My higher, immutable, unsurpassed nature
Commentary: Not knowing my higher nature as the Supreme Self, the ignorant
think that I have just now come into manifestation, having been
unmanifested hitherto, though I am the ever luminous Lord. To what is
their ignorance due ?—Listen
We must also know next verse to know proper meaning.
7.25. I am not manifest to all, veiled (as I am) by Yoga-Maya. This deluded world knows not Me, unborn and imperishable.
Commentary: I am not manifest to all people ; that is to say, I am manifest only to a few who are my devotees. I am veiled by Yoga- Maya.
Yoga- Maya is the Maya which is none other than the Yoga or union of
the three gunas.—Or, Yoga is the firm will of the Lord or Isvara. The
Illusion or veil thereby spread is called Yoga-Maya.—Wherefore people
are deluded and know Me not as unborn and imperishable. That Yoga-Maya
by which I am veiled and on account of which people do not recognise
Me, is Mine, i. c, subject to My control, and, as such, it cannot
obstruct My knowledge— the knowledge of the Isvara, of the possessor
(or wielder) of the Maya, just as the glamour (maya) caused by a
juggler (mayavin) does not obstruct his own knowledge. .
In 7.24, Krishna (Brahman) says His highest form or unmanifest form is not known to ignorants and also He says his true devotees only knows His manifest form and deluded people don't know His highest form, in next verse. This doesn't contradict World being unreal.