Is there no roof cavity?

Are you a tennant / leasing?

The challenge comes from the isolated frontage (no connection to wall or ceiling) where our cashier is, and the phone line (ADSL) comes up from the floor which cannot be used for fibre cable. 

So far, I suspect the solutions are either power line adapters or a client mode AP or a router in bridge mode? 

You have mentioned earlier about the difficulty of running fibre to your point of sale area. You dont have to run fibre to that point. The ONT can stay where it is and you simply run cat cable to the POS. I understand the 'isolation challenges', but sureley there must be a duct in the POS area where both the power and the telephone line come up from under ground?  You might find you can simply pull though another cable?  Or even re-purpose the current phone cable which quite possibly could be a cat cable. 

A others have mentioned, find a good data/cabling person. 

Edit, you could always install a celing to counter conduit (maybe through a nice made to fit stanless steel pipe). That can carry the cat cables down to the POS.  You could always hang an A1 poster board on it and use it for instore marketing.....digital screen even. 

Thanks to everyone for your advice. I certainly learnt a lot from this experiment.

The idea of putting a powerline adapter in between a ONT and a modem came from the Chorus guy, which I wasn't convinced either. After I got a pair of these adapter from a forum member, I tested it out myself.

With the adapter bridging the ONT and our modem, I got 15 Mbps on my cell phone via Wifi (placed next to the modem on a 100/20 fibre plan). After moved the adapter out and connected the modem straight into ONT, I got 65 Mbps on Wifi and 90 Mbps on laptop.

So, yes it's doable but it's a terrible idea. The adapter between ONT and the modem cripples the whole system speed, but in my current set up it's only the EFTPOS machine that's on the other side of the adapter faces the bottleneck (not really as it doesn't require speedy response I guess).

Again, thanks for the advice and kind support from the forum. Case closed.