One radiator is hot other radiators are cold

This troublesome radiator seems to be sludged up, if this is indeed the only radiator that does not get up to temperature then you will need to turn off the radiator valves, drain this rad then remove the radiator outside and run a garden hose through it to remove any debris inside, refit in reverse order. Also heating system may require re-balancing.

If your boiler is new, get the people who installed it to come back. The problem with your radiator sounds like it may be caused by sludge/debris in the system. If it is, then the system hasn't been flushed properly which it should have been before fitting the new boiler. If it is sludged up, then the next thing to fail will be your boiler and no manufacturer will honour a warranty if the fault is caused by incorrect installation i.e failure to flush the system properly! Hope this helps

If your system needs cleaning due to sludge = which is iron oxide, you would be much better having your system properly power flushed with chemicals and external magnets that catch 80% of the iron oxide, the sludge doesn't just lay in the bottom of the radiators it also sticks to the inside of pipe work I.e weather it's copper or plastic.