MikroTik

We have follwing setup
netmeatal with 120 sector
ip 192.168.245.51

netmetal with 120 sector
ip 192.168.245.90

netmetal with 120 sector
ip 192.168.245. 91

SXTHG
ip 192.168.245.96---------------link--------------------------------SXTHG ip 192.168.245.66 @ ISP

from ISP we got intenet at SXT HG. all connection from 4 device in 10/100 switch. how we can use VLAN in this case to manage traffice at base station putting RB 750 instead of switch

Don't need additional 750 device you can do that width existing devices add 750 in this configuration it is overkill.
You must better explain what do you want to achieve but from this point of view 750 is not needed.

i am facing some strange problem when i ping form 192.168.245.66 to x.x.x.90 some time i get request time out. but client connected to this sector ping contionusly this is bridge network. some time i get RTO to x.x.x.51 also
reply for 8 time then RTO for 2 times...

help me