Funny Internet you have. I mean, my ISP also provides me with Internet via Mikrotik hardare, buti n my case it Ends as a Ethernet cable from the uplink he installed. It is in your case more like you have a Wireless Login...
can I share this connection within the access point. My access point
is TP-Link TL-MR3020
RTFM. If the Access Points can do this, it will lbe in the Manual.
MOST LIKELY though - you have to acutally learn how Networks operate and install a router (mikrotik has nice cheap ones) that acts as the "ISP SIDE" one Computer, and then internally distributs IP addresses and uses NAT to route all the traffic out.
A cheap MIkrotik of the 750 series can do that.
According to the Review a http://www.expertreviews.co.uk/wireless-routers/1290157/tp-link-tl-mr3020
for your device:
The TP-Link MR3020 is a compact 3G router that lets you share your
wireless 3G dongle with more than one Wi-Fi-enabled device
This really is a Point for you to pull out the documentation and read it.
Voting to Close- this i - btw. - off Topic on ServerFault as this is not "in a professional capacity". Home Networks fall into Superuser.