There is guide in pinned thread in this section, you need a pin header to solder to pcb and no you don't need to drill holes, if you clean them from solder good enough to push through pin header you then just solder pin header from the other side of pcb. USB uart can be bought on ebay for as little as 3-5 pounds, then you just need software from pinned thread, connect uart to pcb, follow instructions, 5min and job done
. Open eci modem and see if you can do it, is not that bad. If you will have issues on the way I can assist. If you decide that you can't do this, I could do for you but my spare time for such activities is limited to weekends only as kids
and full time job keep me busy during the week. Don't really want you to end up without a modem for week or so.