OK I will answer all your Question in Three Phases
The first Question was
Is it true that the Quran is a miracle of eternity?
Yes it is a continues miracle for all century.
That it is because from Allah meaning and word with Arabic language and challenge Humans to make one like it:
Allah said قُلْ فَأْتُوا بِكِتَابٍ مِّنْ عِندِ اللَّهِ هُوَ أَهْدَى مِنْهُمَا
أَتَّبِعْهُ إِن كُنتُمْ صَادِقِينَ (سورة القصص، 49)
said bring to me any book from Allah better than it if you say the truth
and make it simple "قُلْ فَأْتُواْ بِعَشْرِ سُوَرٍ مِّثْلِهِ مُفْتَرَيَاتٍ "
(سورة هود، 13)
said bring to me only 10 chapters like this Quran
and lastly make it most simple just one in the same quality as Quran Ayah
فَأْتُواْ بِسُورَةٍ مِّن مِّثْلِهِ "
(سورة البقرة، 23)
and this challenge for all creatures Humans and Devils
"قُل لَّئِنِ اجْتَمَعَتِ الإِنسُ وَالْجِنُّ عَلَى أَن يَأْتُواْ بِمِثْلِ هَـذَا الْقُرْآنِ لاَ يَأْتُونَ بِمِثْلِهِ وَلَوْ كَانَ بَعْضُهُمْ لِبَعْضٍ ظَهِيراً "
(سورة الإسراء، 88)
**no one and no one will ** win this challenge for that it is miracle
Then you asked
What are the signs of miraculousness of the Qur'an now?
for now the challenge also their for Arabic and non Arabic to made book like Quran .
What is extraordinary in this book that I can now understand and wonder now?
For those century their more challenges for them in their Sciences because many Quran Talk about Theories science before it been made
example for that Allah said
Allah said : ( أولم ير الذين كفروا أن السماوات والأرض ``كانتا رتقا ففتقناهما وجعلنا من الماء كل شيء حي أفلا يؤمنون ( 30 ) )
means that the earth and sky was one thing and Allah separate the which this theory is the big explosion was in the Quran before those modern century.
Lastly you ask
If this is true, how can I today understand this miracle and believe in the religion of Islam by the Qur'an?
This answer for this question be simple when you see to Quran as miracle book from Allah and truth that if it from him it say and order me what I can do and what it possible.