What are demerits of technical resignation

As per your post, the new employer gave you time up to 5th August 2019. But you got relieved only on 8th of August 2019. Already the extension period is over. What is the action you have taken for this?
Immediately you contact the employer and tell them that you were relieved onlly on 8th August and hence you couldn't join on 5th August. You can also tell him that before joining you have to make some arrangements for shifting and for which you can ask some time and ask them to extend the last date for joining. Once they say what is the maximum possible time based on that you can give them a letter asking for an extension.
But what I suggest you go to the new employer immediately and tell them that you got relieved on 8th and join immediately. Once you join there within the time frame they have given you service continuation will be there. Then you can apply for some days leave and go and complete your works. That will be a better option. Already you got an extension from them and hence I suggest this to you.

Sir the usual date for joining was 5th august.. I wrote them for extension which has been granted for 2monthd that is in October....
As I was with MHA they relieved me on 8th august with TECHNICAL RESIGNATION.

So can I join on October or has to immediately join as it is a case of TR

When you move from one Govt department to another by resigning in the former one then technical resignation is given so that you can get advantage of your earlier service duration to be added to your next job and you get all the benefits accordingly at the time of your retirement. This is a big facility for the Govt employees. As you have mentioned that you were supposed to join on 5th August but were relieved on 8th August and meanwhile you had taken permission for extension for joining time for 2 months which means the new joining date is 5th October or earlier. So, if you have already taken a permission from the competent authority as such there seems no problem in joining the new post later but not after 5th October.

It is clear that you will get your salary in the new post only after joining there and to that extent you will lose some money financially during this gap period.

Despite your best attempt to join in time by tendering resignation, the process has been delayed and you have been relieved on 8 th of Aug where as the joining date was scheduled on 5 th of August. In the meanwhile, you got an extension of two months in connection of your joining it means you can join by 7 th of October.
In the event of joining within the stipulated period as indicated above, there is no harm since you have been allowed to do so.
However, I am looking the same at a different angle. You were in a government department previously and moving to a government department and if you can maintain continuity of service period, so that there is no breakage of service between the two, you will enjoy all sorts of facilities without having interruption in between. In that way, your seniority won't be affected despite joining a new department and even your promotion in the new department will take place considering your date of joining of the first department. Your PF accumulation is not going to be discontinued under that event.
You will be deprived of all the privileges, if you fail to maintain the continuity of the period.
Since you have been given two month extension, you can join your job during the permissible period except the deprivation of the privileges as stated above.

What is your exact question? Do you want to know about the technical resignation, or, do you want to know how can you join your new organization after the stipulated time? After reading the entire question, I feel that although the heading indicates something else, you want to join your new organization after the stipulated time. You can inform your new organization and intimate them a tentative date of your joining. In more than 99% cases of regular appointment, such request is acceded to if the new appointee joins within one year, or so. But in your letter, try to give proper justification for your proposed late joining. Please note that you will also lose your seniority if you join after 1 st July 2020.